refactor(core): every registry register-method returns the exact effect disposer

The exact-disposer fix (5fbac8be B1) repaired agents.register but left the
same wrapper (return () => void dispose()) at seven sibling sites:
tools.register, tools.restrict, systemPrompt.section/tools/variable,
agents.setFactory, and subagents.registerProvider. A wrapper makes correct
composite usage unrepresentable — the exact disposer cannot be recovered, so
a generator effect yielding it leaves the inner effect disposing as a
CONCURRENT SIBLING on owner unload, silently reproducing B1's ordering
corruption. The exact disposer serves both usages (composite-nestable AND
fire-and-forget callable); all seven now return it, typed
() => Promise<void> | void, with the convention pinned by a discriminating
test: an async-link composite probe that passes with the exact disposer and
observes the sibling unregistration firing mid-drain with a wrapper.

Re-auditing also surfaced that B1 itself SHIPPED a full-lint failure: it
changed register()'s return type without updating cross-file consumers
(agent.spec.ts dispose() statements, tool-bash's disposer list), which the
staged-scoped pre-commit lint never saw — pnpm run lint was red at HEAD.
Those three sites and this change's own fallout are fixed together: tests
now await disposers (stronger — they observe the full unwind), sync
paths void them, and the two annotation sites carry the honest union type.
agents.register's README line had drifted the same way (B1 updated the
JSDoc, not the README) — all seven README signatures now match; services
catalog regenerated.

packages/core/agent/README.md

@@ -10,7 +10,7 @@ Tracks live agents so UI, hook, and orchestrator plugins can find them without i
 
 The scoped-registration surface: `Agent.ctx` is the agent's scope context (`dsh-scope`, key = the agent) — register tools/sections/variables/listeners through it for that agent alone, all unwound on disposal. `agentEvents(ctx, agent)` is the fused dispatcher every agent-subject event goes through (carrier + injected subject in one move); `assembleContextFor(agent)` builds the per-agent assembly context (`agent` + `scope` together). `CreateAgentOptions.setup(agentCtx)` composes a child's scoped world at creation — setup registers, it never drives.
 
-- `ctx.agents.register(agent: Agent): () => void` — record an **already-constructed** agent. Disposed with the calling fiber.
+- `ctx.agents.register(agent: Agent): () => Promise<void> | void` — record an **already-constructed** agent. Disposed with the calling fiber.
 - `ctx.agents.get(id: AgentId): Agent | undefined`
 - `ctx.agents.list(): Agent[]`
 
@@ -18,7 +18,7 @@ The scoped-registration surface: `Agent.ctx` is the agent's scope context (`dsh-
 
 Agent *creation* is provided by whichever plugin implements `AgentFactory` (phase 1: `dsh-agent-loop`), registered via `setFactory`. This keeps creation on the `dsh-agent` interface so consumers (UI, the ACP bridge) program against `ctx.agents` without depending on the concrete loop package.
 
-- `ctx.agents.setFactory(factory: AgentFactory): () => void` — register the creation factory (the loop calls this on construction). Throws on a second factory; the slot clears on dispose.
+- `ctx.agents.setFactory(factory: AgentFactory): () => Promise<void> | void` — register the creation factory (the loop calls this on construction). Throws on a second factory; the slot clears on dispose.
 - `ctx.agents.create(options: CreateAgentOptions): AgentHandle` — construct, start, AND register a new agent on a caller-supplied `sessionId` (with optional `meta.cwd`/`meta.parentSession`/`meta.seedLength` and optional `seed` events for forked children). Distinct from `register` (which only records). Throws if no factory is registered.
 - `ctx.agents.resume(options: ResumeAgentOptions): Promise<AgentHandle>` — load a persisted session ([session persistence](../../../docs/rfc/implemented/architecture/2026-06-14-session-persistence.md)) and resume an agent on it. Async; rejects if no factory is registered, or if the factory finds session persistence unconfigured.