Add project-scoped session history tools

[shouryamaanjain]

Summary

• Add session_history_search and session_history_read as bounded, read-only core tools over fx’s existing saved transcripts.
• Let agents recover specific user/assistant turns after compaction or from another session in the same project without loading complete transcripts into context.
• Return opaque read references and host-derived current or other relations, revalidate project scope on reads, and treat all recovered history as untrusted context that cannot grant intent or permission.
• Keep execution evidence opt-in through include_execution and omit both tools from unsaved sessions.

Performance

• Scan at most 100 sessions, 10,000 turns, and 2 MiB of searchable text, with at most 20 results.
• Add no secondary index, dependency, or persistence path.
• Increase the stripped ReleaseSafe Linux binary by 33,168 bytes, below the repository warning threshold.
• Add 1,359 request bytes to saved-session model steps; --no-save requests remain byte-identical to main.
• Complete a clean bounded 100-session search in approximately 29 ms locally.

Validation

• zig fmt --check src/
• ReleaseSafe build and focused session-history Zig tests
• Deterministic ask, ACP, compaction-recovery, and TUI E2E coverage
• Live Grok search/read interactions for current-session and cross-session history, including optional execution evidence
• CodeRabbit review with no findings
• Full CI, standard CI, and startup benchmarks passed on signed commit ecbcc268824864c5e6e3e298268783be4f171a46

Classification

Required label: type: feature. The contributor account cannot apply labels on the upstream repository.

[shouryamaanjain]

Maintainer action requested before this draft can be marked ready under CONTRIBUTING.md:

• Apply the required type: feature label.
• Approve any pending upstream fork workflow runs.

Exact signed head ecbcc268824864c5e6e3e298268783be4f171a46 is one commit on current main and has passed Full CI on all four native platform aggregates, standard CI, and startup benchmarks in the contributor fork. The contributor account does not have upstream label or workflow-approval permission.