feat(tasks): add task PR link command

[HonestMajority]

Summary

• add agman link-pr <task-id> <PR URL or number> to persist task PR metadata in TaskMeta.linked_pr
• support number-only PR references by resolving the task repo origin remote, plus targeted --from-sidecar repair for legacy .pr-link files
• update PM and engineer prompts so PR linking is explicit in the new task flow
• enforce overwrite protection by canonical PR URL, with --force required for different PRs even when PR numbers match

Validation

• cargo fmt -- --check
• cargo clippy --all-targets -- -D warnings
• cargo test
• git diff --check
• isolated temp-HOME CLI smoke for agman link-pr repo--branch 42 --author alice verifying meta.json and task-info