fix: harden workstream session routing fallback

get-shit-done/references/workstream-flag.md

@@ -24,6 +24,41 @@ GSD now prefers a session-scoped pointer keyed by runtime/session identity
 or the controlling TTY). This keeps concurrent sessions isolated while preserving
 legacy compatibility for runtimes that do not expose a stable session key.
 
+## Session Identity Resolution
+
+When GSD resolves the session-scoped pointer in step 3 above, it uses this order:
+
+1. Explicit runtime/session env vars such as `GSD_SESSION_KEY`, `CODEX_THREAD_ID`,
+   `CLAUDE_SESSION_ID`, `CLAUDE_CODE_SSE_PORT`, `OPENCODE_SESSION_ID`,
+   `GEMINI_SESSION_ID`, `CURSOR_SESSION_ID`, `WINDSURF_SESSION_ID`,
+   `TERM_SESSION_ID`, `WT_SESSION`, `TMUX_PANE`, and `ZELLIJ_SESSION_NAME`
+2. `TTY` or `SSH_TTY` if the shell/runtime already exposes the terminal path
+3. A single best-effort `tty` probe, but only when stdin is interactive
+
+If none of those produce a stable identity, GSD does not keep probing. It falls
+back directly to the legacy shared `.planning/active-workstream` file.
+
+This matters in headless or stripped environments: when stdin is already
+non-interactive, GSD intentionally skips shelling out to `tty` because that path
+cannot discover a stable session identity and only adds avoidable failures on the
+routing hot path.
+
+## Pointer Lifecycle
+
+Session-scoped pointers are intentionally lightweight and best-effort:
+
+- Clearing a workstream for one session removes only that session's pointer file
+- If that was the last pointer for the repo, GSD also removes the now-empty
+  per-project temp directory
+- If sibling session pointers still exist, the temp directory is left in place
+- When a pointer refers to a workstream directory that no longer exists, GSD
+  treats it as stale state: it removes that pointer file and resolves to `null`
+  until the session explicitly sets a new active workstream again
+
+GSD does not currently run a background garbage collector for historical temp
+directories. Cleanup is opportunistic at the pointer being cleared or self-healed,
+and broader temp hygiene is left to OS temp cleanup or future maintenance work.
+
 ## Routing Propagation
 
 All workflow routing commands include `${GSD_WS}` which: