Add first-class display-name authoring without changing agent identity

[myobie]

Problem

Current main (c6846f6) already has the correct runtime primitive: st2 agents reads the first non-empty line of <agent-dir>/name on every invocation and exposes it as name in human and JSON output. Editing that file is therefore display-only and takes effect without touching the declaration, stable identity, inbox routing, task ID, or live process.

The missing piece is a supported authoring surface. Today an operator has to know the private catalog layout and edit or remove name by hand; no CLI verb writes or clears it.

Required contract

• Add an explicit command that sets or clears the optional display name for one exact catalog agent.
• Resolve the target by the Agent Spec's explicit host and identity, independent of folder spelling, and fail closed on missing or ambiguous targets.
• Atomically write only <agent-dir>/name, or remove only that file when clearing. Preserve agent.kdl, status, resources, inbox, archive, context, and links.
• A successful change is visible on the next st2 agents read. It does not reconcile, restart, rename a PTY, mutate a task fingerprint, or notify the live agent.
• The bus identity remains the only routing, addressing, state-anchor, and lifecycle identity. A display name is never an alias and need not be unique.
• Define stable validation for empty input, control characters, line breaks, and a bounded length; emit machine-readable changed/unchanged/error output.
• Keep the operation complete with an ordinary local or synchronized catalog folder; no CAS service is required.

Acceptance

• Set, replace, clear, and idempotent-repeat cases are covered against a plain temporary catalog.
• Human and --json roster output change while identity, task IDs, declaration bytes, and all durable resource bytes remain unchanged.
• A live synthetic agent is not restarted or signaled.
• Missing, retired, remote, and ambiguous target behavior is explicit and fail-closed.

Relationship to existing work

• No rename verb: an agent's identity is its directory name, so renaming means abandoning its inbox, archive, and context #21 concerns renaming durable identity and preserving history. This issue deliberately avoids rename/alias semantics.
• Decide ownership of stable agent identity at PTY creation #64 concerns stable identity at PTY creation. Display name is mutable presentation metadata and cannot become identity authority.
• PR docs(vrs): define Agent Spec field-change reconciliation #102 is frozen field-change VRS work; this command writes host-local catalog state outside Agent Spec fields and must not require editing that draft.

[myobie]

The sibling name-file contract proposed here is superseded by #128: positional Agent Spec identity remains the stable automation key, while optional Agent Spec presentation metadata is mutable and non-authoritative. Draft PR #133 is the current implementation surface. Closing this superseded contract; no stable identity rename or compatibility alias is implied.