docs(agents): document durable session-provenance files

[EricGrill]

Fixes #35.

AGENTS.md only documented .research-session.json for --min-time runs, omitting the durable provenance files (.session-events.jsonl, .session-checkpoint.json) and .thesis-session.json. Portable agents following the single-file protocol therefore left behind incomplete provenance, which breaks /wiki:audit's ability to classify runs as replayable rather than partial.

What changed

Three minimal edits to AGENTS.md, all aligned with the canonical Claude source (claude-plugin/skills/wiki-manager/references/research-infrastructure.md and references/audit.md):

1. Topic-wiki tree (around line 46) — adds .research-session.json, .thesis-session.json, .session-events.jsonl, .session-checkpoint.json with one-line comments distinguishing ephemeral from durable.

2. "Session Registry" paragraph (around line 236) — replaced with a fuller "Session Files" block that covers:

   • Both ephemeral files and both durable files
   • Lifecycle (start / round / completion / interruption)
   • Resume protocol (active ephemeral first, then checkpoint + event-log tail)
   • Stale-session warning at >7 days

3. Audit verdicts section — adds a one-line "Provenance classification" note documenting that audit reads the durable files to classify runs as replayable vs partial.

Verification

./tests/test-plugin-validate.sh   →  70 passed, 0 failed
./tests/test-structure.sh         →  92 passed, 0 failed
./tests/test-codex-sync.sh        →  Codex plugin mirror is in sync
./tests/test-opencode-sync.sh     →  OpenCode plugin mirror is in sync

Pre-flight checklist

• Structural tests pass locally
• No edits under `claude-plugin/skills/wiki-manager/` (sync scripts not needed)
• No command additions/removals (no SKILL.md or README command-table update needed)
• AGENTS.md is the only change, and it now matches the canonical Claude source
• No new lint rules
• No hand-edits in `plugins/llm-wiki/` or `plugins/llm-wiki-opencode/`

Notes for review

• I deliberately mirrored the wording style already in AGENTS.md (dense paragraphs, bold lead-ins) rather than introducing a new format. Word count goes from 1 paragraph to 5, but each paragraph carries information that was missing.
• Did not port the full JSON schemas from `research-infrastructure.md` — those belong in the deeper reference doc, not the portable single-file protocol. AGENTS.md just describes shape and lifecycle.
• This PR is independent of docs(contrib): activate CI, add .github scaffolding, README Develop section #46 (CI activation + .github scaffolding). Either can merge first.