install: symlink global AGENTS.md (auto-load pi-extensions skill at session start)

Add pi-global-AGENTS.md, symlinked by install.sh to ~/.pi/agent/AGENTS.md
(pi's global-instructions file, loaded at every startup). Directs the agent
to read the pi-extensions skill at session start and carries a core
fork/recall cheat-sheet, since on-demand skill description-matching was
leaving the extensions under-utilised.

Symlinked (not cp'd) like keybindings.json: pi never rewrites AGENTS.md, and
the entrypoint re-runs install.sh on every container start, so the directive
self-heals across container recreation / volume wipes.

pi-global-AGENTS.md

@@ -0,0 +1,27 @@
+# Global agent instructions
+
+## Session start: load the pi-extensions skill
+
+If the `fork` and/or `recall` tools are present in your tool list (you are
+running inside the **pi** harness with the pi-fork / pi-observational-memory
+packages), **read `~/.agents/skills/pi-extensions/SKILL.md` before doing any
+non-trivial work.** These extensions are routinely under-utilised when left to
+on-demand description matching; reading the skill up front fixes that.
+
+Core triggers (cheat-sheet — the skill has the full guidance):
+
+- **Fork** (`fork(task=..., effort=fast|balanced|deep)`) when a subtask needs
+  reading many files you won't keep, runs in parallel, or is a well-scoped
+  one-shot whose detail would pollute the main thread. Tiers: `fast`=haiku
+  (mechanical/lookups), `balanced`=sonnet (default: exploration/impl/test),
+  `deep`=opus (architecture, security, ambiguous debugging). Always state
+  decision authority, pass verified context, specify the deliverable, ask for
+  an "unsure about" section. Don't fork trivial or iterative work.
+- **Recall** (`recall(<12-char-hex-id>)`) before a load-bearing action (edit
+  code, ship a change, assert a fact) that rests on a `[high]`/`[critical]`
+  observation or a reflection you did not produce this turn. The compaction
+  summary is lossy by design; one recall is cheap, redoing finished work is not.
+  Not a search tool — you must already have the ID.
+
+For depth on tier selection, fork-brief design, boundary discipline, and the
+observational-memory model, read the full skill.