docs: fix drift against main batch (SSH sidecar, proposed config, provenance, forked build)

DRIFT-STALE fixes (stale/misleading text corrected):
- README.md:160 — "on native Linux it does nothing" → accurate always-render
  description (sidecar written on every OS; jump block still omitted on Linux)
- AGENTS.md:22 — setup-lan-access.sh "no-op on native Linux" → corrected to
  always-render + removed now-redundant inline ControlPath/UserKnownHostsFile
  sentence (that's what the sidecar does, not what the script detects)
- CHANGELOG.md:594 (v2.1.1 intro) — "no-op" → "(see v2.2.0)" historical note
- docs/plan-lan-access-and-pi-extensions.md:222 — annotated the completed
  checkbox with the v2.2.0 refinement

GAP additions:
- README.md:212 — custom config section now describes opencode.jsonc.proposed
  sidecar: written-on-diff, removed-on-match, never overwrites, one-line hint
- README.md build-args table — INSTALL_MEMPALACE_TOOLKIT row now mentions
  MEMPALACE_TOOLKIT_REPO alongside MEMPALACE_TOOLKIT_REF
- README.md (after build-args table) — two new sections:
    "Building a fork / relocated build" — MEMPALACE_TOOLKIT_REPO build-arg
    table + two-step docker build example + credentials note
    "Build provenance (labels + manifest)" — docker inspect + run examples,
    OCI label set, ground-truth manifest path

Version string drift: no hard-coded 1.17.7 found outside CHANGELOG (all
occurrences are historical release entries — correct by definition).
CI-internal items (resolve-versions, check-base-hash.sh): no existing doc
described these mechanics, so no update needed.

CHANGELOG.md

@@ -591,7 +591,7 @@ Container-level rebuild on opencode `1.15.13` (unchanged) and pi `0.78.0` (uncha
 The container can now reach LAN peers that the **host** can reach, regardless of host OS — addressing the macOS/Docker-Desktop limitation where a container in the Linux VM cannot see the host's directly-attached LAN.
 
 - New `rootfs/usr/local/lib/opencode-devbox/setup-lan-access.sh`, invoked (non-fatally) by `entrypoint-user.sh` on every start.
-- **Detection:** on VM-backed hosts (macOS OrbStack / Docker Desktop, Windows Docker Desktop — detected via `host.docker.internal` resolution) it generates a writable `~/.ssh-local/config` that uses the host as an SSH **jump**. On native Linux Docker (LAN reachable directly) it is a **no-op**.
+- **Detection:** on VM-backed hosts (macOS OrbStack / Docker Desktop, Windows Docker Desktop — detected via `host.docker.internal` resolution) it generates a writable `~/.ssh-local/config` that uses the host as an SSH **jump**. On native Linux Docker (LAN reachable directly) the jump block is omitted, but the sidecar is still written for the ControlPath redirect (see v2.2.0).
 - **Mechanism, not policy:** ships a generic `host` (alias `mac`) jump entry + a generated jump key in the writable `~/.ssh-local/` sidecar (necessary because `~/.ssh` is bind-mounted read-only). Your own targets stay in your bind-mounted `~/.ssh/config` (add `ProxyJump host`), pulled in via `Include ~/.ssh/config`.
 - New env knobs: `DEVBOX_LAN_ACCESS` (`auto`|`jump`|`off`, default `auto`), `HOST_SSH_USER`, `DEVBOX_HOST_ALIAS`. When `HOST_SSH_USER` is unset the entrypoint prints the public key to authorize on the host.
 - New `dssh` / `dscp` aliases in `.bash_aliases` (wrap `ssh -F ~/.ssh-local/config`), guarded so they only appear when the jump config was generated.