Use `<context_diff>` instead of `<context-diff>` for instruction deltas
This PR was written by Claude Code on behalf of @DouweM.
Renames the framing tag for diffed instruction updates, added in #8188, from <context-diff> to <context_diff>, so every framing tag uses underscores, matching the released tool_result and assistant_thinking tags. #8188 has not shipped in a release yet (latest is v2.58.0), so this lands before any released history contains the hyphenated form, and there is no need to read the old spelling.
Changes: the producer and the closing-tag escape regex in messages.py, the docs/agent.md description, and the tests' inline snapshots. Python identifiers are unchanged. No cassettes contain the tag.
Follow-up: #10547's FramingTag literal (in _framing.py, not on main yet) needs to use context_diff too.
Refs #8188 (no issue closes).
Checklist
- No breaking changes in accordance with the version policy.
- Any permitted compatibility impact has a label, warning, migration, release note, and exact API-check waiver.
- PR title is fit for the release changelog.
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