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>

scripts/host-config.ts

@@ -89,6 +89,13 @@ export interface HostConfig {
   install: {
     /** How skills are linked into the host dir. */
     linkingStrategy: 'real-dir-symlink' | 'symlink-generated';
+    /**
+     * Instruction-only fallback tier: a committed rules digest this host's
+     * users can hand-copy when there is no full install arm. Delivery is
+     * print-path + user-performed copy ONLY — setup must never write or
+     * overwrite a user's own AGENTS.md. See scripts/gen-agents-digest.ts.
+     */
+    instructionTier?: { rulesFile: string };
   };
 
   // --- Host-Specific Behavioral Config ---