Add source-preserving agent retirement authoring

[myobie]

Outcome

Adds st2 agent retire <host.identity> as an exact, source-preserving lifecycle authoring command.

This PR is intentionally stacked on draft PR #112 because both commands share the new agent authoring surface. It should be retargeted to main after #112 lands.

Contract

• requires the exact explicit <host>.<identity> selector and refuses absent, ambiguous, malformed, or path-derived-only targets
• accepts canonical KDL only and changes only the selected declaration retirement intent to retired #true
• preserves the declaration path, parent, comments, unrelated bytes, adjacent state/resources, and file permissions
• returns distinct authored / unchanged results and always reports runtime retirement as not-observed
• uses a sidecar-free catalog writer fence, atomic same-directory publication, source recheck, file and directory sync
• never creates .retired, moves/deletes source, reconciles, launches, or claims teardown

Verification

• focused authoring unit suite: 8 passed
• focused CLI/live integration suite: 4 passed
• full cargo test --workspace --all-targets -- --test-threads=1: passed
• cargo test --doc --workspace: passed
• scoped Clippy with -D warnings: passed with only the pre-existing eval_run::wait_done too-many-arguments lint allowed
• changed-file rustfmt check: passed
• git diff --check: passed

The live integration launches two independent exec tasks, authors retirement for one exact declaration, proves only that task tears down and GCs, proves the sibling PID remains unchanged, and proves the retired task never relaunches.

Closes #109

[schickling-assistant]

Superseded by the merged reversible desired-state model and source-preserving desired-state authoring.

Posted on behalf of @schickling

field	value
agent_identity	dev3.direct.omp.jmg9x29x
session	dev3.jmg9x29x
agent_persona	generalist
agent_supervisor	unavailable
agent_tool	OMP
agent_tool_version	18.0.11
agent_runtime	OMP 18.0.11
tooling_profile	dotfiles@2161b9c