fix(cogos-harness): restore usage watermark writer + fix anomaly-kinds truncation

[chazmaniandinkle]

Root cause

The cogos-harness plugin migration (#16/#18, landed 2026-08-03) rewrote
user-scope-proprioception.py for portability (CLAUDE_PLUGIN_DATA /
CLAUDE_PLUGIN_ROOT, the session-heartbeat POST). That rewrite dropped the
plan-usage segment and its observatory watermark write -- they only ever
existed in the pre-plugin dotfiles copy
(~/.claude/hooks/user-scope-proprioception.py), which has been unregistered
(no hook wires to it) since the migration. Net effect: the operator's
$COGOS_WORKSPACE/.cog/state/watermarks/claude-usage.json watermark froze at
the migration date and the kernel-vitals probe has been flagging
stale:claude-usage ever since (stale_after=172800s in
kernel-vitals-probe.py's _watermarks()).

What this PR does

Commit 1 ports _official_usage, _session_pct_from_cache,
_weekly_pcts_from_cache, _usage_fields, _usage_seg,
_write_usage_watermark (plus the _read_json_cache_fresh helper and usage
constants) from the orphaned dotfiles copy, verbatim except for the
_DATA_DIR/CLAUDE_PLUGIN_DATA split: the usage caches
(~/.claude/cache/{blocks,weekly,rate_limits}.json) are host-local, not
plugin-scoped, so they stay under Path.home() unconditionally rather than
moving under the plugin's data dir. Wires the segment into the second
proprioception line and the watermark write into the per-turn call path,
matching the pre-migration behavior.

Known limitation, preserved intentionally: the existing
cog-workspace-cwd early-skip is unchanged, so the watermark still will not
refresh from a session whose cwd is inside the cog workspace -- that surface
already gets richer proprioception from the workspace's own
foveated-context.py hook and was out of scope for this port.

Commit 2 fixes an unrelated display bug found while verifying the vitals
line during this work: kernel-vitals-probe.py's _format_line() computed
",".join(anomaly_kinds)[:24], which truncates the joined string, not the
item list -- "degraded,over_budget,quarantined" rendered as
"...degraded,over_budget,qua", a visibly cut-off word. Reproduced live
against the operator's actual .kernel-vitals.json cache content before the
fix. New _join_truncated() stops before any item that would blow the
character budget, so the segment always ends on a whole kind name.

Testing

• python3 -m py_compile on both edited files.
• Exercised _usage_fields / _usage_seg / _write_usage_watermark against
  a scratch rate_limits.json / cache dir (redirected off-host, not the
  operator's real cache) through all three paths: official-source,
  estimate-fallback (official absent + no ccusage caches -> empty segment,
  watermark still written with nulls), and the >=80% official-alarm escalation.
  All three produced the expected fields, segment string, and watermark JSON.
• Exercised _format_line / _join_truncated directly against the exact
  anomaly_kinds list from the operator's live cache
  (["degraded", "over_budget", "quarantined"]); before the fix this renders
  ⚠4anom(degraded,over_budget,qua), after it renders
  ⚠4anom(degraded,over_budget) -- no partial word.

Deployment note

The operator's currently-installed plugin runs from a pinned cache
(~/.claude/plugins/cache/plugins/cogos-harness/0.2.0, gitCommitSha
dabfb36), separate from the marketplace registry clone. Neither is touched
by this PR -- both are deploy artifacts. The fix ships when the plugin is
next updated after this merges. Deployment itself is the operator's call,
not part of this PR.