docs(rfc): fold PR #139's simplification sweep into this set

Deduplicate the parallel sweep from codex/exhaustive-simplification-rfcs
(merged in the parent commit) against the twelve RFCs already here, on
the merits per item:

Ported (rewritten to this set's evidence standard):
- fold-stdio-ui-helper — verified: dsh-stdio-agent is the only runtime
  importer; the fold resolves the documented product-app-depends-on-
  support-package tension. The false acp-agent analogy is replaced with
  the real distinction (protocol product surface vs one app's front-door
  scaffolding).
- generate-rfc-index-tables — verified: the classification RFC records
  rejecting generation; the index tables are the repo's only
  systematically conflicting docs region. Process framing made timeless.

Consolidated into existing owners:
- web searchStatus()/fetchStatus()/WebCapabilityStatus (verified: zero
  production callers; the tool-web README and architecture.md claims are
  drift) → drop-web-providers-change-event, renamed
  drop-unconsumed-web-observation-surface.
- hook/result.durationMs (unread, nondeterministic, normalizer-scrubbed)
  and the double-defaulted defaultTimeoutMs knob →
  tighten-hook-protocol-contract.
- the exercised-but-unadvertised exec.arguments mutation path (a
  tool-bash integration shim rewrites through it) → a sanction-or-seal
  note in the pre-tool-input-rewrite proposal.
- the dormant-guard critique of subagent depth machinery → recorded in
  prune-unimplemented-subagent-vocabulary as the considered-and-rejected
  alternative, with the keep sharpened (uncapped-today acknowledged;
  wiring the cap is the completion, not deletion). getProvider()/list()
  and lastAssistantMessage recorded as examined-and-kept (bash-revert
  precedent; observe-enrich recorded keep).

Not ported (with reasons):
- tools/change + system-prompt/change removal — recorded keeps in the
  adapter-change RFC, unengaged by the sweep; no new facts.
- LlmService.models() — flagged by both surveys, but two lines with a
  plausible consumer: TODO-or-drive-by territory per the RFC bar, not
  a proposal.
- SchemaProp.default RFC — already XXX(unused-default)-tagged; the RFC
  bar excludes TODO-tracked provisional cleanups.
- PreToolDecision 'ask' removal — FIXME(permissions)-anchored deferral
  with the permission system as its named consumer.
- Codex bridge Config.model, merged systemMessages — wire-faithful
  tested surface / README-documented deferral.

Their in-code TODO notes (acp-test-harness, hook-snapshot-noise,
catalog-type-links) and the stale hook-prompt-block name fixes ride the
merge unchanged.

docs/rfc/README.md

@@ -53,7 +53,7 @@ Do NOT write one for a mechanical or local choice (a variable name, a one-file r
 | [Unify the agent id and the session id](proposed/simplification/2026-06-20-unify-agent-and-session-id.md) | 2026-06-20 |
 | [Prune producer-less vocabulary variants (block cache hints, the `agent` message source, the `continuation` turn trigger)](proposed/simplification/2026-07-04-prune-producerless-vocabulary-variants.md) | 2026-07-04 |
 | [Drop `GenerateOptions.prefill` and `ToolSchema.strict` — request knobs with no working end-to-end path](proposed/simplification/2026-07-04-drop-inert-request-knobs.md) | 2026-07-04 |
-| [Drop the unconsumed `web/providers-change` event](proposed/simplification/2026-07-04-drop-web-providers-change-event.md) | 2026-07-04 |
+| [Drop the unconsumed web observation surface — the `providers-change` event and the status methods](proposed/simplification/2026-07-04-drop-unconsumed-web-observation-surface.md) | 2026-07-04 |
 | [Drop the `image` content block until a path can honor it](proposed/simplification/2026-07-04-drop-image-content-block.md) | 2026-07-04 |
 | [Prune write-only fields and a dead routing knob from the fs seam](proposed/simplification/2026-07-04-prune-write-only-fs-surface.md) | 2026-07-04 |
 | [Prune the unimplemented subagent seam vocabulary](proposed/simplification/2026-07-04-prune-unimplemented-subagent-vocabulary.md) | 2026-07-04 |
@@ -61,12 +61,7 @@ Do NOT write one for a mechanical or local choice (a variable name, a one-file r
 | [Prune dead core-spine surface — `SurfaceManager.invalidate()`, the loop-internal exports, `ToolExecutionResult.callId`](proposed/simplification/2026-07-04-prune-dead-core-spine-surface.md) | 2026-07-04 |
 | [Share the app bins' boot glue instead of maintaining twin copies](proposed/simplification/2026-07-04-share-app-bin-boot-glue.md) | 2026-07-04 |
 | [Remove the `agent/steering` mirror emit](proposed/simplification/2026-07-04-remove-agent-steering-mirror.md) | 2026-07-04 |
-| [Tighten the hook-protocol contract — the `native` dialect, `suppressOutput`, and lib-owned `hook/result` semantics](proposed/simplification/2026-07-04-tighten-hook-protocol-contract.md) | 2026-07-04 |
-| [Narrow the subagent seam to synchronous collect](proposed/simplification/2026-07-04-narrow-subagent-synchronous-collect.md) | 2026-07-04 |
-| [Drop idle registry and status observation surfaces](proposed/simplification/2026-07-04-drop-idle-registry-observation-surfaces.md) | 2026-07-04 |
-| [Remove defaults from the tool-schema DSL](proposed/simplification/2026-07-04-remove-tool-schema-defaults.md) | 2026-07-04 |
-| [Trim unused hook protocol and bridge surface](proposed/simplification/2026-07-04-trim-hook-protocol-surface.md) | 2026-07-04 |
-| [Narrow the pre-tool gate to shipped behavior](proposed/simplification/2026-07-04-narrow-pre-tool-gate.md) | 2026-07-04 |
+| [Tighten the hook-protocol contract — dialect, discarded fields, double defaults, and lib-owned `hook/result` semantics](proposed/simplification/2026-07-04-tighten-hook-protocol-contract.md) | 2026-07-04 |
 | [Fold the stdio UI helper into the stdio app](proposed/simplification/2026-07-04-fold-stdio-ui-helper.md) | 2026-07-04 |
 
 ### Architecture