feat: require target claims in handoff

[kaleab-kali]

Summary:

• Add zaphod handoff --require-target-claim for dirty-worktree-safe target reservation checks.
• Keep target claim readiness in the handoff JSON and reuse existing claim_required/claim_conflict errors.
• Document the agent workflow and command reference behavior.

Tests:

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features --target-dir target\handoff-target-claim-full-gate3 -- -D warnings
• cargo test --all-targets --all-features --target-dir target\handoff-target-claim-full-gate3
• git diff --check
• target\handoff-target-claim-full-gate3\debug\zaphod.exe handoff --help