feat(hosts): instruction-only tier — a 2KB committed rules digest any agent host can read

New agents-digest/gstack-AGENTS.md (1,765 bytes, hard 2,048-byte budget):
gstack's ethos one-liners, the reuse ladder, and voice rules for hosts with
no install arm — Zed, Amp, Jules, or any AGENTS.md-reading agent. Generated
by scripts/gen-agents-digest.ts, auto-refreshed by gen:skill-docs, committed
like llms.txt so setup's explainer arms can point at it before any toolchain
exists. First line carries the gstack version as its own staleness nudge.

Delivery is print-path + user-performed copy ONLY: setup never writes or
overwrites a user's AGENTS.md (a test pins this — no cp/ln/mv/redirect into
AGENTS.md anywhere in setup). openclaw and hermes explainer arms print the
path; slate keeps routing to the full Claude install and gbrain ships from
its own repo. HostConfig gains the optional install.instructionTier slot,
declared by both instruction-tier hosts. README host table now matches what
setup actually does.

Inspired by dietrichgebert/ponytail's instruction-tier AGENTS.md fallback —
one generated source, never per-host hand copies.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

hosts/hermes.ts

@@ -28,6 +28,13 @@ const hermes = defineHost({
     // If Hermes has GBrain as a mod, brain features activate automatically.
   ],
 
+  // No full install arm — users can hand-copy the instruction-only digest
+  // (setup's explainer arm prints this path; never auto-copied).
+  install: {
+    linkingStrategy: 'symlink-generated',
+    instructionTier: { rulesFile: 'agents-digest/gstack-AGENTS.md' },
+  },
+
   coAuthorTrailer: 'Co-Authored-By: Hermes Agent <agent@nousresearch.com>',
 });