feat: require target claims in preflight

[kaleab-kali]

Summary:

• Add preflight --require-target-claim for startup target branch reservation checks.
• Preserve current-claim error priority before target-claim errors.
• Document the new guard and cover success, missing, conflict, and priority paths.

Verification:

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features --target-dir C:\tmp\zaphod-preflight-target-full-gate -- -D warnings
• cargo test --all-targets --all-features -j 1 --target-dir C:\tmp\zaphod-preflight-target-full-test
• git diff --check
• C:\tmp\zaphod-preflight-target-full-test\debug\zaphod.exe preflight --help