Add first-class display-name authoring

[myobie]

Summary

• add st2 agent name <identity> DISPLAY_NAME and --clear as the supported display-name authoring surface
• resolve the target from explicit Agent Spec host/identity instead of folder spelling, accepting a bare identity only when unique
• atomically change only the adjacent name file and return stable changed/unchanged/error JSON receipts
• keep qualified remote and retired declarations authorable because display metadata has no routing or runtime authority
• document the identity boundary and fail closed on missing, ambiguous, invalid, or unsafe targets

Acceptance evidence

The executable acceptance covers set, replace, clear, idempotent repeat, human and JSON roster projection, path-independent resolution, ambiguity, missing targets, validation, retired/remote targets, and non-regular name paths. It snapshots declaration/status/inbox/archive/context/link bytes and resolved task IDs, and keeps a live synthetic child unchanged and unsignaled throughout authoring.

Validation

Exact head: 65b637fcff6981270c29ff16475ef3413e990f60

• cargo test --workspace --all-targets -- --test-threads=1
• cargo test --doc --workspace
• targeted Clippy with -D warnings (allowing only the pre-existing eval_run.rs too-many-arguments baseline)
• changed-file rustfmt check with module traversal disabled
• git diff --check
• complete five-file diff walk

Draft for review. No merge or rollout.

Closes #108.

[schickling-assistant]

Superseded by KDL name/description fields and the merged rename/describe authoring surface.

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