Base: add gitleaks; surface git-crypt in smoke + docs

Both tools are used as part of the secret-management setup in several
of the repos this devbox operates on (gitleaks pre-commit hook +
git-crypt for selectively-encrypted canonical config). Having them in
the container means hooks fire correctly inside instead of warning
'gitleaks not installed' on every commit.

git-crypt was already installed via apt in Dockerfile.base (line 58),
just unasserted by smoke and unmentioned in user-facing docs.

gitleaks is new: Go-compiled binary fetched from GitHub releases via
the same /releases/latest redirect-resolution pattern as gosu, fzf,
git-lfs, etc. Arch suffix is 'x64' (not 'x86_64' / 'amd64') on this
project — flagged in the Dockerfile comment and in AGENTS.md's
floated-binaries gotcha list.

Adds ~21 MB to the base layer (gitleaks 8.30.1 binary). No variant
threshold bumps needed (2500–3700 MB envelope, 21 MB is noise).

CHANGES

Dockerfile.base — new GITLEAKS_VERSION=latest ARG + install RUN
right after the git-lfs block. Multi-arch (linux/amd64=x64,
linux/arm64=arm64). Echoes resolved version + runs 'gitleaks version'
to fail the build on any install error.

scripts/smoke-test.sh — git-crypt and gitleaks added to the
'Resolved component versions' table (printed first thing in CI logs)
and to the 'Core binaries' assertion list (run helper). Smoke now
fails fast if either binary regresses.

README.md — 'What's in the image' tree line names gitleaks alongside
the existing git-crypt.

AGENTS.md — gitleaks added to the 'GitHub-sourced binaries float by
default' list with a new clause flagging project-specific arch-name
deviations (gitleaks=x64, bat/eza/zoxide=x86_64/aarch64, gosu=
amd64/arm64). Saves the next person from the 'why does this not
download' debugging session.

CHANGELOG.md — sub-entry under existing Unreleased, before the
PI_VERSION/OMOS_VERSION cache-hit fix entry.

DOWNSTREAM IMPACT

This is a base-layer change — base-decide will compute a fresh
base-<hash>, build-base will run (no cache hit), all four variants
will rebuild. First real base rebuild since v1.14.50b. Pi-devbox's
next FROM base-latest pull picks up gitleaks automatically with no
Dockerfile change there.

Verified end-to-end on host: gitleaks 8.30.1 21 MB binary extracts
cleanly from the URL the Dockerfile constructs and 'gitleaks version'
prints '8.30.1'.

Holding off on tagging — opencode + pi upstreams unchanged at 1.15.10
and 0.75.5 respectively. Will ride along with the next upstream-bump
release rather than burning a base rebuild on a no-upstream-change
container-only roll.

AGENTS.md

@@ -70,7 +70,7 @@ cd /tmp && npm pack @earendil-works/pi-coding-agent@0.75.5 && tar -xzf earendil-
   Release-day checklist: README → (regenerate DOCKER_HUB.md only if HUB_TEMPLATE changed) → promote CHANGELOG Unreleased → grep AGENTS.md for stale counts → commit → tag → push tag.
 
   **Between releases the same coupling applies.** Doc drift is not just a release-day concern — a workflow tweak, entrypoint change, or `generate-config.py` refactor can leave any of these four files lying. Before committing a non-release change, grep the docs for references to what you touched: `git diff --name-only HEAD | xargs -I{} grep -l 'thing-you-changed' README.md AGENTS.md DOCKER_HUB.md .gitea/README.md .env.example`. If a doc says "four variants" / "two phases" / "runs on amd64 only" and your change made that no longer true, fix it in the same commit.
-- **GitHub/Gitea-sourced binaries float by default** — gosu, fzf, git-lfs, nvim, bat, eza, zoxide, uv, gitea-mcp, Go, oh-my-opencode-slim all default to `latest`. Each build-time install step reads the `/releases/latest` Location redirect (or the go.dev JSON feed for Go) and derives the concrete version. Use the same `ARCH` case-switch pattern for multi-arch support (amd64/arm64). Intentional pins: `OPENCODE_VERSION` (drives the image tag), `NODE_VERSION=22` (major pin), `DEBIAN_VERSION=trixie-slim` (OS base). Adding a new upstream tool: follow the existing floated-version pattern, don't hardcode a specific tag.
+- **GitHub/Gitea-sourced binaries float by default** — gosu, fzf, git-lfs, gitleaks, nvim, bat, eza, zoxide, uv, gitea-mcp, Go, oh-my-opencode-slim all default to `latest`. Each build-time install step reads the `/releases/latest` Location redirect (or the go.dev JSON feed for Go) and derives the concrete version. Use the same `ARCH` case-switch pattern for multi-arch support (amd64/arm64) — mind project-specific arch-name deviations (gitleaks uses `x64`, bat/eza/zoxide use `x86_64`/`aarch64`, gosu uses `amd64`/`arm64`). Intentional pins: `OPENCODE_VERSION` (drives the image tag), `NODE_VERSION=22` (major pin), `DEBIAN_VERSION=trixie-slim` (OS base). Adding a new upstream tool: follow the existing floated-version pattern, don't hardcode a specific tag.
 - **Resolved versions are logged by the smoke test** — `scripts/smoke-test.sh` prints a "Resolved component versions" table as its first step. CI logs always capture what got baked into a given image even when ARGs default to `latest`.
 - **`PI_VERSION` and `OMOS_VERSION` MUST be passed by CI as concrete versions**, not left at the `latest` default. The npm install steps in `Dockerfile.variant` (`npm install -g @earendil-works/pi-coding-agent` / `oh-my-opencode-slim@${OMOS_VERSION}`) produce identical layer-hashes when the ARG values are byte-identical across builds; combined with the registry buildcache (`base-buildcache`) the layer gets reused even when `latest` would have resolved to a newer upstream. This is the same class of bug that bit pi-devbox v0.74.0 → v0.75.5 (silent same-bytes-across-releases regression discovered 2026-05-23, fixed in pi-devbox v0.75.5b). It is currently *masked* in opencode-devbox by `OPENCODE_VERSION` being a hard-coded ARG that bumps every release — that bump invalidates the parent-chain cache key for the downstream pi/omos layers — but the masking would fail the moment a `vN.N.Nb` opencode-version-unchanged release ships that only bumps pi or omos. Preventative fix: `.gitea/workflows/docker-publish-split.yml` has a `resolve-versions` job that runs `npm view @earendil-works/pi-coding-agent version` and `npm view oh-my-opencode-slim version`, exposing concrete values as outputs that every variant smoke + build job consumes via build-args. Smoke tests assert via `EXPECTED_PI_VERSION` / `EXPECTED_OMOS_VERSION` env vars — would catch the regression on the next release rather than four releases later. **If you change the variant build-args list, the resolve-versions job, or the smoke EXPECTED_*_VERSION wiring, audit all affected jobs in lockstep.**
 - **Shell scripts use `set -euo pipefail`** — both entrypoints are strict. Errors in volume chown or SSH permission operations are intentionally suppressed with `|| true`.