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>

.changeset/2937-statusline-context-position.md

@@ -0,0 +1,5 @@
+---
+type: Added
+pr: 0
+---
+**Context-window meter position is now configurable via `statusline.context_position`** — set `"front"` to render the meter immediately after the model name (useful in narrow terminals where the right edge is clipped); the default `"end"` preserves the existing byte-identical output. Invalid values at `config-set` time are hard-rejected by the enum validator; at hook runtime an invalid/stale config silently falls back to `"end"` so the statusline is never broken. Closes #2937.