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>

sdk/src/query/config-schema.ts

@@ -63,6 +63,7 @@ export const VALID_CONFIG_KEYS: ReadonlySet<string> = new Set([
   'hooks.workflow_guard',
   'workflow.context_coverage_gate',
   'statusline.show_last_command',
+  'statusline.context_position',
   'workflow.ui_review',
   'workflow.max_discuss_passes',
   'features.thinking_partner',