fix: give the statusline channel reading a session dimension (#1362)

[fdaviddpt]

Closes #1362.

Problem

scripts/statusline.py's cache is keyed on the repository alone, and for almost every field (prs, issues, latest, pr_checks, etc.) one shared reading per repository is exactly right. The channel field is not such a fact: raw_state's forwarding vs not_subscribed distinction comes from channel:health's own session: row -- whether the session that took the reading is subscribed to the socket. Two sessions on one repository (one armed via bin/oss-workspace, one a bare claude subscribed to nothing) can hold genuinely different, simultaneously correct answers, and whichever session's reading landed last was rendered as every session's own state -- a measurement taken by a different session, rendered as this session's own, which is this repository's own defect class landing on the instrument whose job is telling unknown from known.

Fix

• channel_status() gains a fifth cannot_determine reason, other-session, via two new optional kwargs (session, current_session). Checked right after the existing not-asked check and before attribution/staleness -- a reading from a different session is untrustworthy regardless of how sound it otherwise looks.
• The cache document's channel dict gains a session field, written by refresh() (new optional session_id param) on a fresh reading, threaded from gather()'s own payload.get("session_id") (Claude Code's own statusline JSON payload) through _fork_refresh's new --session-id argv.
• Every other cached field stays shared exactly as before -- only the channel reading, whose correct answer genuinely differs by session, gained the new dimension. A cache written before this fix carries no session key and self-heals at the next refresh, the same convention statusline: a repo that must declare a short watch_name can never render anything but ch? #754's own attribution migration used.

Self-review finding, fixed in place

The self-review Explore spawn found that a malformed session_id (a non-string value from a malformed payload) would reach subprocess.Popen's argv unchecked and raise an uncaught TypeError, crashing the entire statusline render rather than costing only the channel field its answer. Fixed at both the write site (_fork_refresh only forwards a truthy str) and the read site (gather() coerces a non-string value to None at the source), with a new red/green test. fix_commit_scope.py reported within-scope, so no second review round was needed. The oss:auditor spawn's four-class checklist (absence handling, guard-not-run, untrusted-text boundary, cross-platform) returned clean.

Tests

New file tests/test_statusline_channel_session_1362.py: must-fire/must-not-fire pairs for the other-session reason at both the channel_status() unit level and the gather() integration level, a backward-compat control for a cache with no session key recorded, refresh()/_fork_refresh()/main() argv-threading tests, and the self-review's non-string-session_id regression test. Six pre-existing test files were touched only to widen a monkeypatched _fork_refresh lambda's signature to accept the new optional third parameter.

Not pushed, not opened as a PR by this session -- both commits are on fix/1362, ready for the maintainer to push and open.

🤖 Generated with Claude Code

https://claude.ai/code/session_01QYm4rkixzFWu3MXm2r9hhG

[AI-generated]