Support Anthropic preserved-thinking requirements

Claude Fable 5.1 binds preserved thinking to the model and the exact `system`, `tools`, and messages that preceded it.
Changing that prefix can cause a `400 invalid_request_error`. [Anthropic preserved-thinking migration guide](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking)

[#11022](https://github.com/aaif-goose/goose/pull/11022) established the foundation by making normal Goose message assembly append-only for prompt caching.
It stopped moving turn context between requests and added prefix-stability tests.

The remaining gaps are history rewriting, system prompt changes, tool changes, model switching, and mismatch handling.

### Tracked work

#### 1. Remove ongoing tool-pair summarization ([#11764](https://github.com/aaif-goose/goose/issues/11764))

- [ ] Complete [#11764](https://github.com/aaif-goose/goose/issues/11764), which removes a background rewrite of earlier tool messages.
https://github.com/aaif-goose/goose/pull/11817

#### 2. Keep the system prompt stable

- [ ] Deliver subdirectory hints discovered mid-session as an agent-only message instead of rewriting `system` (#11284, #10330).

Hints are the only mid-session `system` rewrite left. Mode changes and the timestamp never reach the shipped template. Extension toggles are tracked under 3.

#### 3. Make tool changes append-only

- [ ] Keep `system` and `tools` prefix-stable when an extension is enabled or disabled mid-session (#11905).

Confirmed under enforcement: enabling an extension mid-round grows `tools` in place and drops the block.

#### 4. Add Fable 5.1 enforcement and recovery

- [x] Add Fable 5.1 model support (#11836)
- [x] Test prefix mismatch behavior (verified live on Fable 5.1 with `block_binding`)
- [x] Define recovery for prefix mismatches: `drop_block` by default, one retry with thinking history stripped on a signature 400 (#11836)

#### 5. Keep the assistant turn in model order

- [x] Stop relocating streamed thinking onto the tool-call message in both loops (#11837)

#### 6. Smaller prefix rewrites to close after the above

- [ ] `enrich_unclaimed_tool_errors` rewrites earlier tool results per request (unrolled loop).
- [ ] Unanswered tool requests are stripped from an earlier assistant message after a cancelled tool round.
- [ ] Compaction must land between turns, never with a tool_use awaiting its result.
- [ ] Unknown response block types are dropped from the replay.
- [ ] Default effort sends `thinking: disabled` to Opus 5, so binding is never attached by default.

#### Findings from the first implementation pass

- The most common failure had nothing to do with prefix edits: Opus 4.7+ returns thinking blocks with empty text plus a signature, and goose dropped them on parse, so every tool round replayed the turn without its reasoning. Fixed in #11836.
- Fable 5.1 also binds each signature to the earlier blocks in the same assistant message. Both agent loops reordered `[thinking, text, tool_use]` to `[text, thinking, tool_use]`, which drops the block on every tool round. Fixed in #11837.
- Enforcement is opt-in for accounts created before 2026-08-31. Goose now opts in with `drop_block` so breaks are visible in `input_transformations` rather than silent.
- Mid-conversation `role: "system"` is accepted by Opus 5, Sonnet 5, Opus 4.8, and Fable; 4.7 and older return 400.