Split execution/evidence/ into distinct sub-packages (recording/, session-log cluster)

[Trecek]

Summary

resolve-review on PR #4947 (the #4672 execution/ decomposition) found that execution/evidence/ bundles three unrelated concerns, purely because #4672 was a mechanical move driven by a file-count architecture gate (REQ-CNST-010), not a domain redesign:

1. Session-log cluster (durable, cross-run history): session_log.py, _session_log_recovery.py, _session_retention.py, session_index.py
2. Runtime observability signals (always-on): anomaly_detection.py, linux_tracing.py, otlp_sink.py
3. Recording/replay tooling (env-var-gated, AUTOSKILLIT_RECORD_SESSION only): recording.py, _recording_skills.py

Two independent DISCUSS-investigation passes (PR #4947 local resolve-review) confirmed clusters 1 and 3 are self-contained enough to split into their own sub-packages, leaving cluster 2 as a coherent "observability" package with a natural one-line inclusion criterion.

Proposed changes

• Split recording.py + _recording_skills.py out into a new execution/recording/ sub-package.
• Split the session-log cluster (session_log.py, _session_log_recovery.py, _session_retention.py, session_index.py) into a new sub-package with a distinct name from the existing execution/session/ — reusing "session" would conflate this durable-log concern with execution/session/'s actual meaning (single-run result processing: _exit_classification, _session_outcome, _retry_fsm, etc.). Candidate: execution/session_log/ (not yet bikeshedded — track the naming choice via Tracking: Naming inconsistencies for a future consistency pass #4552).
• After both splits, execution/evidence/ keeps only the observability trio and its AGENTS.md can state the unifying criterion ("runtime signals collected for post-hoc diagnosis") directly, instead of describing a heterogeneous grab-bag.

Compatibility-sensitive surface

Touches the execution/__init__.py and execution/evidence/__init__.py re-export gateways, covered by the test_gateway_reexports_pre_move_names / REQ-GATEWAY-PARITY contract tests in tests/contracts/test_package_gateways.py. Also touches execution/evidence/AGENTS.md and any direct-submodule imports in tests (e.g. tests/execution/test_session_log_retention.py imports _session_retention/_session_log_recovery directly).

Source

PR #4947 local resolve-review round — cohesion findings, DISCUSS verdicts (indices 28, 30, 52), independently re-investigated by a separate sub-agent pass before this ticket was filed. See also #4552 (naming-consistency tracker) for the sub-package naming discussion.