ci(gate): fewer full rebuilds: accept a clean, disjoint base; no full clean in a private build dir

[andersalm]

Problem. The local pre-push gate forced a full rebuild far more often than needed:

• It refused any branch that does not contain the newest develop (ci-local-prepush.py:119). So every merge into develop sent every open branch through another 10–15 minute check.
• Before every compile it ran cargo clean -p for all repository packages, because the worktrees share one build directory.

Change.

1. A branch based on an older develop is accepted when its merge with develop is clean and no path changed on the branch was also changed on develop. Otherwise the old refusal stays. The merged result is proven by the eight required PR checks (develop requires them; PR CI runs on the merge ref), as GitHub's non-strict protection already allows.
2. The full clean is skipped only when the build directory is inside this worktree and is not the shared default target-build. Remote build boxes use this.

Tests. ci-local-prepush-test.py: 62 pass.

Written by Claude Opus. Reviewed by GPT-6 Sol: MODIFY (no unchecked override; say CI proves the merge), done.

[andersalm]

Verdict record: candidate 3dd877c (head) = cf5d70ab + 9f6bb3c1, rebased with no change to the reviewed bytes. Review by GPT-6 Sol: MODIFY (drop the override; say CI proves the merge), fixed in 9f6bb3c1. ci-local-prepush-test: 62 OK.
CI: the first run failed in an unrelated flaky Rust test (update_controller second_owner_action…, being fixed); the rerun is green.

[andersalm]

Candidate ed914a4: merges develop e844fb5 into the reviewed 3dd877c. Delta: the conflict at ci-local-prepush.py:554. Kept this PR's private build-dir rule; dropped the two disk_reserve calls that #234 removed. ci-local-prepush-test: 61 OK.