feat(debug): add reverse time travel

[pocky]

Summary

• Add rr-backed reverse debugging for the debug extension, including record/replay, reverse stepping, watchpoints, trace listing/deletion, and origin-finding workflows.
• Gate rr-specific MCP tools behind [debug.record] backend = "rr" so unsupported or unconfigured hosts do not advertise unavailable reverse-debug capabilities.
• Persist and prune trace metadata with bounded retention, cleanup handling, and structured unsupported/error payloads instead of transport failures.
• Expand user-facing docs, protocol tests, fixture coverage, and e2e/functional tests for the new reverse-debug workflow.

Changes

Configuration and Runtime Gating

• .tower/config.toml: Add local [debug.record] rr trace retention and recording timeout settings.
• crates/core_engine/src/adapters/config/debug.rs: Parse, validate, serialize, and test [debug.record] configuration with rr-only backend support and safe relative trace directories.
• crates/core_engine/src/adapters/daemon/engine.rs: Filter rr-specific debug tools from the bundled manifest unless rr recording is configured.
• Makefile: Reorder default extension build list to keep the debug extension included in default extension installs.

Debug Extension

• extensions/debug/extension.toml: Advertise rr-specific record/replay, reverse execution, watchpoint, trace, and origin tools.
• extensions/debug/src/main.rs: Initialize rr runtime support, route rr tool dispatch, and preserve fixture behavior for reverse-debug tests.
• extensions/debug/src/protocol.rs: Add record/replay configuration, schema declarations, and rr tool protocol metadata.
• extensions/debug/src/rr.rs: Add rr preflight detection, bounded record subprocess execution, fixture recording support, output capture, timeout handling, and trace registration.
• extensions/debug/src/traces.rs: Add trace identifiers, metadata persistence, retention policy, pruning, lookup, completion, and safe deletion.
• extensions/debug/src/session.rs: Add replay-backed session support, reverse continue, step back, watchpoints, replay seek targets, and replay session metadata.
• extensions/debug/src/tools.rs: Add public MCP handlers and DTOs for record, replay, reverse_continue, step_back, watchpoint, traces, delete_trace, find_origin, and record_and_find_origin.
• extensions/debug/src/origin.rs: Add origin-finding workflows over replay sessions, including watchpoint-driven reverse execution and record-and-find orchestration.
• extensions/debug/src/eval_at.rs: Share captured-variable serialization helpers with origin capture while preserving eval-at behavior.
• extensions/debug/src/types.rs: Add structured runtime failure DTOs for tool-facing error payloads.

Tests and Fixtures

• crates/core_engine/tests/extension_debug_e2e.rs: Add rr tool declaration, gating, DTO, cleanup, fixture-backed reverse-debug, real-rr skip/pass, and parallel stress coverage.
• crates/core_engine/tests/functional_debug_reverse_mcp.rs: Add functional MCP coverage for public record/replay/origin workflows and structured no-prior-write results.
• crates/extension_protocol/tests/protocol_tests.rs: Update protocol expectations for the expanded debug tool catalog and rr gating behavior.
• extensions/debug/tests/eval_at_contract.rs: Preserve eval-at captured-variable contract coverage after helper extraction.
• extensions/fixtures/debug_adapter/src/main.rs: Add scripted record/replay fixture scenarios, deterministic reverse-debug events, cleanup tokens, and side-channel cleanup reporting.
• extensions/fixtures/debug_adapter/tests/eval_at_scenario.rs: Add fixture scenario selection, cleanup, ordering, and JSON event contract coverage.

Documentation and Project Knowledge

• AGENTS.md: Document rr-backed reverse-debug MCP tools and their configuration-dependent availability.
• README.md: Update feature and documentation summaries for rr-backed reverse debugging.
• docs/README.md: Refresh documentation index entries for reverse-debug support.
• docs/extensions.md: Document debug extension reverse-debug behavior and tool exposure.
• docs/getting-started.md: Add rr record/replay setup guidance.
• docs/mcp-tools.md: Add MCP reference entries for rr record/replay, reverse execution, trace, and origin tools.
• docs/user-guide/debug-sessions.md: Add user guidance for rr-backed reverse-debug workflows.
• .zpm/kb/architecture/journal.wal: Record architecture decisions and conventions for debug eval-at and reverse-debug implementation context.
• .zpm/kb/feedback/journal.wal: Record reusable feedback rules for trace deletion, rr output capture, and reverse-debug cleanup tests.

Test plan

• Run cargo fmt --check
• Run cargo clippy --workspace --all-targets -- -D warnings
• Run cargo build --workspace --bins
• Run cargo test --workspace

Closes #12

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