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/gen-agents-digest.ts

@@ -0,0 +1,89 @@
+#!/usr/bin/env bun
+/**
+ * Generate agents-digest/gstack-AGENTS.md — the instruction-only tier.
+ *
+ * A ~2KB behavioral digest any rules-reading agent host can load (Zed, Amp,
+ * Jules, Cursor side-projects, OpenClaw/Hermes methodology mode): the ETHOS
+ * principles as one-liners, the reuse ladder, the voice rules, and a pointer
+ * to the full install. It is a COMMITTED generated artifact (like llms.txt)
+ * so the setup explainer arms can point at it before any toolchain exists.
+ *
+ * Delivery is print-path + user-performed copy ONLY — never write or
+ * overwrite a user's own AGENTS.md (a gstack-AGENTS.md is auto-read by no
+ * host; clobbering AGENTS.md destroys user content on every re-run).
+ *
+ * One host-neutral file, deliberately: the digest contains no host-specific
+ * paths, so per-host copies would be byte-identical duplication (the exact
+ * per-host rule-text sprawl gstack's generator model exists to avoid). If a
+ * host ever needs rewrites, this generator gains a per-host emit then.
+ *
+ * Size budget: <= 2,048 bytes, enforced by test/agents-digest.test.ts. Every
+ * host that loads this pays for it in every session — trim, don't grow.
+ *
+ * Refresh: invoked from scripts/gen-skill-docs.ts after SKILL.md generation.
+ */
+
+import * as fs from 'fs';
+import * as path from 'path';
+
+const ROOT = path.resolve(import.meta.dir, '..');
+export const DIGEST_RELPATH = path.join('agents-digest', 'gstack-AGENTS.md');
+export const DIGEST_BYTE_BUDGET = 2048;
+
+export function generateAgentsDigest(opts?: { root?: string }): { content: string; bytes: number } {
+  const root = opts?.root ?? ROOT;
+  const version = fs.readFileSync(path.join(root, 'VERSION'), 'utf-8').trim();
+
+  const content = `# gstack digest v${version} — regenerate/re-copy after upgrading gstack
+
+Behavioral rules from gstack (https://github.com/garrytan/gstack), compressed
+for agent hosts without a full skill install. The full skills add workflows,
+reviews, and evals on top of these rules.
+
+## Ethos
+
+- **Boil the Ocean** — AI makes completeness cheap, so do the complete thing: tests, edge cases, error paths. Shortcuts need an explicit, recorded decision.
+- **Search Before Building** — know what exists before deciding what to build. Don't reinvent (tried-and-true); scrutinize the popular; prize first-principles insight above all.
+- **User Sovereignty** — models recommend, the user decides. Cross-model agreement is signal, never permission. Ask before changing the user's stated direction.
+- **Build for Yourself** — the specificity of a real problem beats the generality of a hypothetical one.
+
+## The reuse ladder
+
+Before writing new code, stop at the first rung that holds:
+1. A helper, util, or pattern already in this repo.
+2. The standard library.
+3. A native platform feature (CSS over JS, DB constraint over app code).
+4. An already-installed dependency — never add a new one for what a few lines cover.
+
+Then build the complete version of what remains. Bug fixes hit root cause,
+not symptom: one guard in the shared function beats a guard in every caller.
+
+## Voice
+
+Direct, concrete, builder-to-builder. Name the file, function, command, and
+user-visible impact. Short paragraphs; end with what to do. No filler, no
+corporate tone, no AI vocabulary.
+
+## Full gstack
+
+Clone https://github.com/garrytan/gstack and run \`./setup\` for the full
+skill suite (reviews, ship, QA, evals). This digest is generated — edit
+scripts/gen-agents-digest.ts, not this file.
+`;
+
+  return { content, bytes: Buffer.byteLength(content, 'utf-8') };
+}
+
+export function writeAgentsDigest(opts?: { root?: string }): { outPath: string; bytes: number } {
+  const root = opts?.root ?? ROOT;
+  const { content, bytes } = generateAgentsDigest({ root });
+  const outPath = path.join(root, DIGEST_RELPATH);
+  fs.mkdirSync(path.dirname(outPath), { recursive: true });
+  fs.writeFileSync(outPath, content);
+  return { outPath, bytes };
+}
+
+if (import.meta.main) {
+  const { outPath, bytes } = writeAgentsDigest();
+  console.log(`[gen-agents-digest] ${path.relative(ROOT, outPath)}: ${bytes} bytes (budget ${DIGEST_BYTE_BUDGET})`);
+}