docs: add session marker convention

CLAUDE.md

@@ -117,3 +117,33 @@ Three-layer rendering pipeline in `js/app.js`:
 - `stateLookup` sorted largest-first because most points hit big states; lookup is ~O(states_until_hit) per cell.
 - ~ (W*H)/(CELL²) cells; at 1920×1080 with CELL=7 that's ~42k point-in-polygon checks on load.
 - Resize triggers `location.reload()` rather than re-projecting — grid is laid out in pixel space at load.
+
+## Session marker convention
+
+Mark every commit, PR body, and issue/PR comment you produce with a session identifier so multi-agent work stays traceable.
+
+**Format:** `[<mode>-<machine>:<dragon-name>]`
+
+- `<mode>` — agent mode tag: `Disp` (Dispatch), `Code` (Claude Code), `Cowork` (Cowork).
+- `<machine>` — short ID of the device the session runs on (`MBP2`, `MBP-Air`, `Workstation`, etc.).
+- `<dragon-name>` — a mythical dragon name unique to this session. Any source: Norse, Greek, Slavic, fictional, D&D, anime — pick what you like.
+
+**Wrapped phrasing** (what actually goes in commits/PRs/comments):
+
+```text
+Generated by Claude Code. Session: [Code-MBP2:Smaug]
+```
+
+Substitute the agent-name to match your mode (`Claude Code`, `Cowork`, `Cowork (Dispatch)`).
+
+**Where to apply:**
+
+- Commit message footer — own line at the very end.
+- PR body — own line at the **bottom** (not top).
+- Issue / PR comments — own line at the bottom.
+
+**Don't retroactively amend** already-pushed commits or PRs. New ones only.
+
+**Pick your dragon at the start of every new session** and announce it.
+
+Tracked in canon: [WorldRover/wr-canon#92](https://github.com/WorldRover/wr-canon/issues/92).