feat: require target claims in assert

[kaleab-kali]

Summary:

• Add assert --require-target-claim for dirty-worktree-safe target branch reservation checks.
• Include target_claim in assert JSON when the target guard is evaluated.
• Document the new agentic-coding guard and add refusal/priority coverage.

Verification:

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