Harden Discord session thread reconnect

[shiny-code-bot]

Summary

• add session: <session_id> to new Discord session start markers so future reconnects have a stable identity beyond pid
• preserve exact matching for legacy start markers without the session line
• allow pid-relaxed reconnect only for new session-id-bearing markers, and only when exactly one candidate matches
• add diagnostics for pid-relaxed reuse, ambiguity, and no-match cases

Validation

• uv run python -m unittest tests.test_every_code -q
• uv run ruff format --check discord_blue/doodads/every_code/bridge.py discord_blue/doodads/every_code/threads.py tests/test_every_code.py
• uv run ruff check discord_blue/doodads/every_code/bridge.py discord_blue/doodads/every_code/threads.py tests/test_every_code.py
• uv run mypy discord_blue/doodads/every_code/bridge.py discord_blue/doodads/every_code/threads.py tests/test_every_code.py
• git diff --check

Review agents first flagged the wrong-thread risk in pid-relaxed matching; this version requires the persisted session id before relaxing pid and was re-reviewed as safe to merge.