docs: update AGENTS.md structure + ARCHITECTURE.md see-also for pi bring-up

AGENTS.md Structure block was stale (listed only 2 bin/ wrappers, no
extensions/, no contrib/). Added full tree + a new 'What install.sh does'
section enumerating all steps, gates, and probes so maintainers see at a
glance what the installer touches.

ARCHITECTURE.md is scoped to the producer side (feeding the palace);
pi extension is consumer side, so out of scope for the main body. Added
a pointer in the See also section so readers can find extensions/pi/README.md.

ARCHITECTURE.md

@@ -364,5 +364,6 @@ When #1 merges, retire `mempalace-docs` to a thin shim. When #2 + #3 land togeth
 - [`README.md`](README.md) — human-facing quickstart + per-tool usage reference.
 - [`AGENTS.md`](AGENTS.md) — repo conventions for AI agents modifying this codebase.
 - [`SKILL.md`](SKILL.md) — agent skill (producer side), symlinked into `~/.agents/skills/opencode-mempalace-bridge/` by `install.sh`.
+- [`extensions/pi/README.md`](extensions/pi/README.md) — pi coding-agent bring-up: the MemPalace MCP bridge extension, mosh-friendly keybindings, settings template for starting pi without `--model`, and the `~/.config/pi/.env` + zsh loader pattern for AWS env vars. Out of scope for this document (which is producer-side feeding), but linked from `install.sh` which handles both.
 - `~/.agents/skills/mempalace/SKILL.md` — agent skill for the **consumer** side (searching, diary, KG). Pair with `SKILL.md` in this repo.
 - [`cli_utils`](https://gitea.jordbo.se/joakimp/cli_utils) — sibling repo: shell quality-of-life tools. Origin of these wrappers before the 2026-04-30 split.