feat: support awaited extension continuity across compaction

[phernandez]

Why

Refs #506. Memory extensions need to finish a checkpoint before context replacement, recover its receipt on the correct branch, and insert recall before the next prompt without triggering another agent response. Basic Memory is the concrete consumer: basicmachines-co/basic-memory#1489.

What Changed

• A single CodingSession helper emits awaited extension start/end callbacks for manual, detailed manual, threshold, and overflow compaction. No-op threshold checks emit nothing; failure/cancellation emits an aborted end.
• context.branch_entries: deep-copied persisted active-path entries, including custom receipts, for reload/resume reconciliation and branch lineage.
• await context.summarize(messages, instructions=..., timeout=60): bounded tool-free synthesis using the active model/provider, without an agent turn or history mutation. Generation guards reject late results.
• await tau.append_message(content, custom_type=...): persist reference context while idle before the next prompt. Queued follow-ups—even with trigger_turn=False—can generate another model response; this method does not queue a turn.
• In-place tree branching emits shutdown/start(reason="branch") on the same runtime. Extensions must support closing and restarting resources.

Design and Boundaries

No persistence destinations, exposed credentials, installed-package patches, or detached summary tasks in core. Extensions select inputs, own cancellation, validate writes and retain receipts. Observation failures preserve the diagnostic-only contract; this is not a compaction veto API.

This addresses extension delivery from #506, not threshold/manual frontend async-iterator or TUI status work, and deliberately does not close that issue. entry_appended delivery is unchanged; persisted active-path inspection supports receipt reconstruction without it.

Verification

• uv run pytest -q: 1881 passed, 2 Windows-only tests skipped on macOS.
• uv run mypy: passed for 121 source files.
• Ruff lint/format: passed.
• Focused session/continuity tests prove a blocked start handler finishes before replacement, receipts persist through all four paths, no-op/cancellation semantics, summary isolation, zero-turn context insertion, and tree lifecycle notifications.
• Downstream Basic Memory integration: 51 tests pass, including actual session compaction/reload/branch/resume, headless Textual /reload, and real Basic Memory transcript/checkpoint persistence in temporary projects. Model providers are deterministic fakes; no production user session or paid model was used.

Risks

Summary calls use the active provider's ordinary routing/billing. Extensions must choose bounded inputs and timeouts and must not treat a queued instruction or diagnostic-only callback as acknowledgement of an external write. References inserted with append_message are persisted custom messages and contribute to normal context size until compacted.

[phernandez]

Superseded by #687, submitted from the Basic Machines organization fork: https://github.com/basicmachines-co/tau. The replacement contains the identical tested commit; please direct review there. This work supports Basic Memory continuity: basicmachines-co/basic-memory#1489.