Convert greybeard to a leaf director; only Skywalker may spawn

[TheGreatAxios]

CL-7670: only the primary orchestrator may spawn; greybeard becomes a leaf.

Converts the limited spawn CL-7662 kept with a recorded reason into a leaf:

• Greybeard: tier leaf, maySpawn false, tools = review allowlist (no fleet verbs). Checklists, self-read, and provenance from CL-7662 untouched.
• Registry test pins Skywalker as the only maySpawn:true closed director.
• Stale nested-orchestrator references (types, tool-sets, docs, fleet/exec tests) updated; tier itself left unused for now.
• Skywalker prompt unchanged: no numeric spawn caps introduced (verified by grep; the one 'at most one explorer' line predates this change on origin/main).

[linear-code]

CL-7670

[TheGreatAxios]

Audit (builder) — HOLD honored, comment-only, no code changes

No merge from this review — the do-not-merge hold stands. No code touched.

Quality: thorough. Checklists, the self-read deviation note, and the CL-7019/CL-7662 provenance carried over with intent intact; the follow-up commit (wording coherence) cleans the leaf language. The registry test now pins Skywalker as the only maySpawn: true closed director, and the agent-fleet/runner tests were updated to match the leaf shape rather than left asserting the old allowlist. Docs (ARCHITECTURE / IMPLEMENTATION / PRODUCT) updated in the same unit.

Capability tradeoff — the real cost of this leaf. Greybeard loses intern/explorer/critic evidence-gathering. A review blocked on a concrete unknown can no longer fetch it in one hop; it must name the unknown under Blockers or bounce back via ask_director, turning an evidence fetch into a parent round-trip. Fine for advisory reviews of familiar code, but expect thinner evidence on unfamiliar ground unless the parent re-fetches. The success_criteria fail-close critic path goes away with it — that language is correctly deleted, not orphaned.

Dead nested-orchestrator tier note. SubagentTier keeps the tier with zero closed directors on it (“no closed director uses this tier today”), and tool-sets.ts keeps the nested-orchestrator surface for the package filter. Either keep it explicitly as the extension point for future nested packages or remove it — a memberless tier invites the next author to re-invent greybeard's old shape. Suggest a one-line decision at merge time, not a third state.

Diff read: gh pr diff 958 (13 files); registry/greybeard/fleet test pins verified against the branch tip.

[TheGreatAxios]

Hold lifted per operator direction: removed the "Review only \u2014 do not merge (0.3.25 posture)." line from the PR body; all other body content unchanged.

No rebase needed \u2014 branch is MERGEABLE/CLEAN against main at 3bf85ec with all 9 CI checks green.

Local verification at tip 3bf85ec:

• bun test src/agent/directors/greybeard/ src/agent/directors/registry.test.ts \u2014 39 pass, 0 fail
• bun run typecheck (tsc --noEmit) \u2014 clean, exit 0

Not merging in this lane \u2014 leaving for the fresh review fleet.