fix: Gate the Actor-not-found recovery hint on the loaded tools (#1302)

Third instance of the #1295 bug (tracked in #1296), this time in the
actors family. Review turned up two more, all fixed here.

Result text is gated by nothing, and `tools.mode_contract.test.ts` scans
`description` only, so none of this was covered.

| Site | Names, ungated | Session that hits it |
| --- | --- | --- |
| `fetch-actor-details` not-found | `search-actors` |
`?tools=fetch-actor-details` |
| `call-actor` not-found | `search-actors` | `?tools=call-actor` |
| `search-actors` footer | `fetch-actor-details` |
`?tools=search-actors`, on every success |

None of those names are in `AUTO_INJECTED_TOOLS`. The first two need
only a typo to reach.

All three are gated in place, so a session holding every tool gets
byte-identical output. Verified against master, not assumed.
`call-actor`'s not-found branch does not reuse
`buildCallFailureRecoveryHint`, because its second offer is a detail
lookup, which for a nonexistent Actor fails the same way.

`fetch-actor-details`'s description also gained the missing "resolve the
name first" instruction. Only the `search-actors` half is gated; the
rest names no tool and renders everywhere.

## This does not fix the guessing habit

`task-chain-hard-1` (#1294's `tasks-evals`): Sonnet 3/3, Haiku ~7/10
before this wording and 5/8 with it. No measurable change. The case is
well-formed and the Haiku gap is a capability limit, so further
rewording is not the lever.

That 5/8 predates the review, which split the sentence, so it does not
describe what ships. Re-running is a one-variable comparison: the case
loads `tasks` + `actors`, so every gate here is a no-op for it.

## Not fixed here

Same bug, unfiled, left out to keep this one thing:

- `actor_tools_factory.ts:124`: Actor tool descriptions name
`call-actor`, and `?tools=<actor>` serves none. `mode_contract`'s stub
description hides it from the sweeping guard.
- `call_actor_widget.ts:96`: names `call-actor` in an apps widget-only
session. That sentence is the whole recovery path, so it needs
replacement wording, not a gate.

1464 unit tests, type-check, lint, format, and `check:agents` clean.
Merges cleanly with #1190 and #1294.

Written and reviewed with Claude Code.

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Co-authored-by: Claude <noreply@anthropic.com>