fix: scope caveman mode to the session

Mode was applied per session but stored in one file per machine
($CLAUDE_CONFIG_DIR/.caveman-active). Four bugs followed from that one
fact: parallel windows shared a mode; deactivation was spelled "no flag
file", so SessionStart re-derived the configured default and undid an
explicit "stop caveman" after every auto-compaction; the statusline badge
rendered whichever window wrote last; and "off" could not survive a
SessionStart at all.

payload's `source`, but it read the legacy flag, where off is absence — a
deactivated session found nothing stored and fell back to getDefaultMode()
regardless. The other three are consequences of the storage shape, not of
when the hook re-derives.

State now lives in .caveman-sessions/<session_id>.mode, keyed by the
session_id Claude Code puts in every hook payload and in the statusline's
stdin JSON. The legacy flag stays as a last-write-wins compat mirror and
never receives the literal 'off': an older hook reading that would inject
"CAVEMAN MODE ACTIVE (off)" and an older statusline would render
[CAVEMAN:OFF]. Both spellings of off are read everywhere — a missing file
(old) and a literal 'off' (new, durable).

caveman-config.js gains the primitives: validateSessionId (a whitelist,
because a session id becomes part of a path), resolveActiveMode,
readSessionModeRaw, writeSessionMode, per-session prev for one-shot skills
(#599), gcSessionStore, and a session_id-tagged recordModeChange (#601).
Every one takes a sessionId that may be null or malformed and degrades to
the previous machine-wide behavior — the old code path IS the fallback
branch, which is why the existing suites, nonid,
pass unchanged.

SessionStart re-derives the default only on `startup` and `clear`; compact,
resume, fork, an unrecognized source and the n`
read the stored value. Grouping `clear` with the resets deliberately
differs from #691's comment: per-session storage makes the distinction
cheap, and nothing else in the conversation survives /clear either. The
watchdog must not reset, or a slow payload on a compaction re-arms a
session the user turned off.

The three entrypoints resolve the new helpers individually instead of
demanding them in their requireSibling shape checks (#848): a
caveman-config.js from before this change satisfies those checks, so
hard-failing would trade "machine-wide mode, no
state at all" on exactly the plugin-cache drift that guard exists for.

Both statusline ports take session_id from the stdin JSON with bounded,
TTY-guarded reads (integer timeout — bash 3.2 rejects fractional read -t)
and render nothing for a deactivated session. verify_repo.py greps them
against SESSIONS_DIRNAME and SESSION_ID_RE, tws
badge. caveman-stats filters the transition l
window's switches no longer skew another's es
(bin/install.js, uninstall.sh, uninstall.ps1) removes the session store;
.caveman-history.jsonl is kept on purpose. opide
behaviour — it writes its own flag in its own config dir and never sees a
Claude Code session_id.

Tests: 25 unit cases in tests/hooks/caveman-config.test.mjs,
SessionStartSourceTests, SessionScopedModeTests, 5 statusline cases, and
tests/manual/session-mode-smoke.sh (22 checks, end-to-end against a
throwaway config dir). docs/testing-session-modes.md carries the full plan
including the live-Claude-Code checks. Hook ced.

npm test 197/197 · tests/*.js clean · unittesss
· smoke 22/22

Signed-off-by: Pavel Filippenko <filippenko.ps@phystech.edu>