feat: live stack-state injection via agent harness hook (#304)

[aarontrowbridge]

Implements #304.

Adds a second opencode plugin (amicode_context.ts) implementing experimental.chat.system.transform that reads live state on every prompt build and injects it as the final system[] entry.

Scope (expanded 2026-08-21): the original WIP moved solver mode + routing off the boot-time file splice. This update finishes the job — the fleet line and the user-memory sections (profile, recent problems, reference demos, mount stack, memory index) also move to the live hook. Motivation: the boot-time splices were the "Amicode forgets the fleet" bug class — a hand-maintained server bundle (the canonical launchd server's staging dir) or a stale re-prep served yesterday's memory index and yesterday's solver mode to every new session (observed: 10-entry index frozen at Aug 16 vs 18 live; cloud-HP solver instructions served after the user switched back to Piccolo).

Key changes:

• opencode-plugin/stack_state.ts — standalone state readers + markdown builders (node-builtins only for Bun runtime): solver mode, routing, active problem, live runs (original WIP) + NEW lean fleet line (~/.amico/ops/fleet/fleet.json role, fleet-status.json devices/freshness with stale/unreadable degradation, on-demand pointers — absent fleet.json renders nothing) + NEW marker-only Armonia mount discovery (smol-toml-free port of mount_store semantics: kind ranks, duplicate-id skip, kind-rank ordering) and the five user-memory sections read live from the personal vault, section text byte-identical to the retired substrate/user_splice.ts builders. No mounts.toml manifest support (smol-toml unavailable in Bun; no manifest exists in practice — documented in-module).
• opencode-plugin/amicode_context.ts — the plugin, single export, one hook (unchanged)
• src/opencode_config.ts — solver/routing AND the user-memory/mount/memory-index splices removed from prepareOpencodeProject (the latter previously the only injector); buildOpencodeConfigContent accepts extraPluginPaths (original WIP)
• src/extension.ts — boot + solver-switch register the context plugin; solver-switch projectDir asymmetry fixed (original WIP)
• packages/extension/AGENTS.md — single broadened recovery pointer (template tail) covering solver mode, routing, fleet, and the vault files; replaces the WIP's programmatic append (was duplicated)
• Deleted substrate/user_splice.ts + the now-dead vault_store index readers (KNOWLEDGE/DEMOS/MEMORY) — the plugin owns that text now, pinned golden in test/stack_state.test.ts (14 cases: fleet rendering + degradation, mount discovery incl. duplicate/marker-less skips, section text parity with the retired builders, caps 50/30/50, splice-parity section order, hermetic env-seam composition). solverModeSection/routingSection deliberately stay as the original WIP's parity oracles (test-only) — noted asymmetry, reviewer's call whether to fold them into stack_state pins too.
• test/packaging.test.ts — pins amicode_context.ts + stack_state.ts as must-ship vsix assets.

Hook support verified against the deployed fork binary: experimental.chat.system.transform is typed (packages/plugin/src/index.ts:291), triggered (agent/agent.ts:381), and the frozen canonical server binary contains the hook string. Silent no-op remains the failure mode if a binary lacks it — which is why the recovery pointer exists.

Deployment sequencing (the canonical launchd server serves a hand-staged bundle):

1. Merge + release a vsix carrying the plugin files (the server config's existing amicode_context.ts reference — dead since it was added — goes live the moment the newest installed vsix ships it).
2. Restart the canonical server (it resolves __EXT__ to the newest vsix at boot).
3. Strip the now-duplicated static sections (About this user / recent problems / reference demos / mount stack / memory index) from the staged ~/.amico/server/opencode-project-staging/opencode-project/AGENTS.md — until then they render twice (harmless, just noisy).

Verification: tsc --noEmit clean; full extension suite 1186 passed / 5 skipped / 100 files.

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