fix(config-get): return schema default for context_window when absent (#2944)

* fix(config-get): return schema default for context_window when absent (#2943)

cmdConfigGet in bin/lib/config.cjs now consults a SCHEMA_DEFAULTS map before
emitting "Key not found", so context_window (and any future schema-defaulted
keys) return their default value (exit 0) when not set in config.json.

Also updates the stale subagent-timeout.test.cjs assertion that expected the
old broken behavior (exit 1 / "Key not found") to match the corrected behavior.

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

* test: use distinct sentinel to prove --default wins over schema default (#2943)

* docs: update CHANGELOG.md for #2943 fix

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -39,6 +39,7 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
   posting a comment that points to the contribution guide. (#2872)
 
 ### Fixed — 1.40.0-rc.1
+- **`config-get context_window` returns `200000` when key absent** — querying an unset `context_window` previously exited 1 with "Key not found", surfacing a confusing error in planning logs even though the workflow fallback worked correctly. `cmdConfigGet` now consults a `SCHEMA_DEFAULTS` map and returns the documented default (`200000`, exit 0) for absent schema-defaulted keys; unknown absent keys still error as before. (#2943)
 - **`gap-analysis` now parses non-`REQ-` requirement IDs and ignores traceability table headers** — `parseRequirements()` no longer hard-codes the `REQ-` prefix and now accepts uppercase prefixed IDs such as `TST-01`, `BACK-07`, and `INSP-04`; markdown table header rows (for example `| REQ-ID | ... |`) are excluded so header tokens are not reported as phantom uncovered requirements. Added regression coverage for mixed-prefix REQUIREMENTS files with traceability tables. (#2897)
 - **Gemini slash commands namespaced as `/gsd:<cmd>` instead of `/gsd-<cmd>`** —
   Gemini CLI namespaces commands under `gsd:`, so `/gsd-plan-phase` was unexecutable.