feat(#2161): opt-in absolute token count on the statusline context meter (#2174)

* feat(#2161): opt-in absolute token count on the statusline context meter

New statusline.show_context_tokens config (default false). When enabled,
the context meter shows the absolute token total after the percentage,
e.g. "████░░░░░░ 46% (156k)" — summing input, cache-creation, cache-read,
and output tokens from context_window.current_usage (matching /context).

Default output is byte-for-byte unchanged when the flag is absent or
false. The .planning config is now read once per render and shared with
the last-command/position block instead of being re-read.

Claude-Session: https://claude.ai/code/session_01Hme55Pvq6BhpgwBcyC5HAg

* docs(#2161): changeset fragment for PR #2174

* fix(#2161): review fixes — k-to-M threshold, boundary tests, changeset format

- formatTokens promotes to the M branch when k-rounding reaches 1000
  (999,500-999,999 rendered "1000k" instead of "1.0M")
- boundary tests at 999499/999500/999999/1000000/1000001
- Number() guards on the four usage fields (silent string-concat gap)
- changeset body ends with the (#2161) citation per house convention

Claude-Session: https://claude.ai/code/session_01Hme55Pvq6BhpgwBcyC5HAg

* fix(#2161): round-2 review fixes — config-set coverage, precision claim, exports style

- config-set accept/reject tests for statusline.show_context_tokens
  (mirrors the post-planning-gaps precedent the issue scope names)
- changeset + docs no longer claim parity with /context: the suffix sums
  four fields while the meter %% derives from used_percentage (three), so
  the figures can diverge slightly
- module.exports one entry per line

Claude-Session: https://claude.ai/code/session_01Hme55Pvq6BhpgwBcyC5HAg

* test: regenerate golden-install-parity fixtures for the statusline hook change

Claude-Session: https://claude.ai/code/session_01Hme55Pvq6BhpgwBcyC5HAg

---------

Co-authored-by: Tom Boucher <trekkie@nomorestars.com>

src/config.cts

@@ -763,6 +763,13 @@ function cmdConfigSet(cwd: string, keyPath: string | undefined, value: string |
   const VALID_CONTEXT_POSITIONS = ['front', 'end'];
   if (kp === 'statusline.context_position') assertEnumValue(parsedValue, val, VALID_CONTEXT_POSITIONS, 'statusline.context_position');
 
+  // statusline.show_context_tokens — boolean only
+  if (kp === 'statusline.show_context_tokens') {
+    if (typeof parsedValue !== 'boolean') {
+      error(`Invalid statusline.show_context_tokens '${val}'. Must be a boolean (true or false).`);
+    }
+  }
+
   // Fallow scope + profile enum validation (#3424)
   const VALID_FALLOW_SCOPES = ['phase', 'repo'];
   if (kp === 'code_quality.fallow.scope') assertEnumValue(parsedValue, val, VALID_FALLOW_SCOPES, 'code_quality.fallow.scope');