refactor(statusline): delegate the althing segment to althing-statusline

althing 3.4.0 ships the segment as a console script on PATH, so the twelve
lines that existed in three hand-maintained copies now exist in one. A
script, unlike a document, has somewhere to be installed, and installing it
makes drift impossible rather than merely visible.

Two real defects left this file with the block, both silent: the handle was
resolved as the most recent launch in a directory, so a directory hosting a
claude and a codex seat reports the codex handle's unread count to a Claude
session as soon as the codex pane relaunches last; and the post-office
address was hardcoded, which survives until the post office moves and then
reports an outage that is really a stale constant.

Taken from the live path rather than pushed to it. ~/.claude/statusline-
command.sh had been migrated directly and was AHEAD of this tracked copy,
with a better `command -v` guard; a reflexive cp from the repo would have
destroyed it. Repo and live are now byte-identical.

The deploy instructions and the v3-cutover history move from the file header
into docs/runbooks/althing-deploy.md, where they cannot drift against the
script they describe, along with the diff-before-you-copy-in-both-directions
warning that this near-miss earned.

docs/runbooks/althing-deploy.md

@@ -272,6 +272,62 @@ and reports it on every run and on `--check` — and **deliberately never sets
 it.** That file is the operator's permission configuration; a deploy script
 that edits it is a deploy script granting itself trust.
 
+## The status-line segment belongs to althing, not to this repo
+
+From althing **3.4.0** the segment is a console script, `althing-statusline`,
+installed on PATH by the same `uv tool install --force` as the other seven
+executables. `scripts/claude-statusline-command.sh` calls it and does not
+reimplement it:
+
+    althing=$(printf '%s' "$input" | timeout 5 althing-statusline 2>/dev/null)
+
+⚠ **Keep that outer timeout LOOSER than the program's own 2 s budget.** If the
+outer one fires first the segment comes back empty, which renders as "not an
+althing directory" — the outage conflation, reintroduced by the guard meant to
+prevent a hang.
+
+**Why a console script rather than the `sync_skill.sh` canonical-file pattern:**
+a script, unlike a document, has somewhere to be *installed*. Installing it makes
+drift impossible instead of merely visible.
+
+**Why it needed solving at all.** On 2026-09-02 the same twelve lines existed in
+three places — althing's `plugin/scripts/statusline.sh`, the operator's wired
+`~/.claude/statusline-command.sh`, and this repo's copy — and one audit found a
+**different defect in each**. All three were fixed separately, by hand, the same
+day. Two of the defects were in this repo's copy and are worth knowing because
+they are both silent:
+
+- the handle was resolved as the most recent launch in the directory, so a
+  directory hosting both a claude and a codex seat (`~/development/Worldtree`)
+  reports the **codex** handle's unread count and reachability to a Claude
+  session the moment the codex pane relaunches last;
+- the post-office address was hardcoded, so it survives exactly until the post
+  office moves and then reports an outage that is really a stale constant.
+
+### Deploying it
+
+`scripts/claude-statusline-command.sh` is the tracked copy of what Claude Code
+actually runs at `~/.claude/statusline-command.sh` (`settings.json` →
+`statusLine.command`). Copies, not symlinks — same rule as `stacks/`.
+
+    diff -u scripts/claude-statusline-command.sh ~/.claude/statusline-command.sh
+    cp scripts/claude-statusline-command.sh ~/.claude/statusline-command.sh
+
+⚠ **Diff before you copy, in both directions.** On 2026-09-02 the live path was
+edited directly and was *ahead* of the tracked copy; a reflexive `cp` from the
+repo would have destroyed the better version. The tracked copy is intent, the
+live path is reality, and reality is sometimes right.
+
+Smoke test — Claude Code runs it in a **bare shell**, so unset the althing
+env vars or you will test your own session's identity instead of the resolution
+logic:
+
+    printf '{"workspace":{"current_dir":"/tmp"}}' \
+      | env -u ALTHING_HANDLE -u ALTHING_POST_OFFICE bash scripts/claude-statusline-command.sh
+
+Want: empty althing segment for a directory with no identity, `🔔` in a seat's
+own directory, `📵` against an unreachable address.
+
 ## Rollback
 
     uv tool install althing-core==3.1.2