Two correctness bugs from @trek-e review:
1. Grep pattern `^<task` only matched unindented task tags, missing
indented tasks in PLAN.md templates that use indentation. Fixed to
`^\s*<task[[:space:]>]` which matches at any indentation level and
avoids false positives on <tasks> or </task>.
2. Threshold=0 was documented to disable inline routing but the
condition `TASK_COUNT <= INLINE_THRESHOLD` evaluated 0<=0 as true,
routing empty plans inline even when the feature was disabled.
Fixed by guarding with `INLINE_THRESHOLD > 0`.
Added tests/inline-plan-threshold.test.cjs (8 tests) covering:
- config-set accepts the key and threshold=0
- VALID_CONFIG_KEYS and planning-config.md contain the entry
- Routing pattern matches indented tasks and rejects <tasks>/</task>
- Inline routing is guarded by INLINE_THRESHOLD > 0
Review feedback on #2061 from @trek-e.
