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>

docs/CONFIGURATION.md

@@ -452,6 +452,7 @@ If `.planning/` is in `.gitignore`, `commit_docs` is automatically `false` regar
 | `hooks.workflow_guard` | boolean | `false` | Warn when file edits happen outside GSD workflow context (advises using `/gsd-quick` or `/gsd-fast`) |
 | `statusline.show_last_command` | boolean | `false` | Append `last: /<cmd>` suffix to the statusline showing the most recently invoked slash command. Opt-in; reads the active session transcript to extract the latest `<command-name>` tag (closes #2538) |
 | `statusline.context_position` | string | `"end"` | Position of the context-window meter. `"end"` (default) renders at line tail; `"front"` renders immediately after the model name so the meter stays visible in narrow terminals. Closes #2937 |
+| `statusline.show_context_tokens` | boolean | `false` | Append the absolute token count (e.g. `(156k)`) after the context meter's percentage. Sums input, cache-creation, cache-read, and output tokens from the hook payload — a broader basis than the meter's percentage (which excludes output tokens), so the two figures can diverge slightly. Opt-in; the meter is unchanged when the flag is absent |
 
 The prompt injection guard hook (`gsd-prompt-guard.js`) is always active and cannot be disabled — it's a security feature, not a workflow toggle.