feat(statusline): add opt-in context_position config for narrow terminals (#2937)

Extract composeStatusline() helper from duplicated inline template logic in
runStatusline() and renderStatusline(). Both call sites now route through the
helper, which accepts a position param ('end' | 'front', default 'end').

- 'end' (default) preserves byte-identical output to v1.38.x and earlier
- 'front' renders ctx immediately after model name, before the first │
- Invalid values silently coerce to 'end' at runtime (belt-and-suspenders;
  config-set rejects invalid values upfront via enum validator)

Adds statusline.context_position to VALID_CONFIG_KEYS in both CJS and TS
schemas, enum validator in config.cjs, docs row in CONFIGURATION.md,
and a changeset. Closes #2937.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/CONFIGURATION.md

@@ -361,6 +361,7 @@ If `.planning/` is in `.gitignore`, `commit_docs` is automatically `false` regar
 | `hooks.context_warnings` | boolean | `true` | Show context window usage warnings via context monitor hook |
 | `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 |
 
 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.