mecatui: /agent command and picker for session-scoped agent definitions

[kantord]

Stage: Implementation (stacked on #2003)

Adds the mecatui side of ADR 0353 so session-scoped agent identity can be tried end to end from the TUI. Stacked on #2003 (base branch impl/session-scoped-agent-identity): review that backend PR first; this diff is only cmd/mecatui/ (15 files, +653/-4).

What this does

• /agent <name> starts a new session bound to the named agent definition (CreateSessionRequest.agent_definition_name). An unknown name fails loudly and recoverably.
• Bare /agent (typed or from the / palette) opens a filterable picker listing definitions by name; Enter starts a session bound to the highlighted one. /agents still shows full details.
• Both send the operator's active model selection like every other create path. Without it a definition with no model: fell to the server default model.
• Submitting /agent closes the slash-command palette.

Verification

• go test ./cmd/mecatui/... and task lint pass. New tests cover parsing, active-selection carry-through (mutation-checked), the picker filter/select/error/stale-result paths, and palette closing (mutation-checked).
• Tried by hand against a local mecated: /agent <name> and the picker create bound sessions.

Known limitations and open questions

• The operator's active model selection overrides a definition's own model: (ADR 0353 case 4: an explicit provider+model pair wins). Whether a definition's pin should win instead is undecided.
• A server older than ADR 0353 that advertises agent definitions would silently ignore agent_definition_name; there is no capability flag for this.
• The picker has no mouse click targets; keyboard and wheel only.
• Scheduled tasks cannot bind to an agent definition (no field on ScheduleSpec); not part of this PR.

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Fully or partially written by an AI agent.

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