feat: support native plans in output modes - #71

Summary

Adds first-class plan rendering to exported HTML while keeping native plan lifecycle state outside print, JSON, and RPC execution protocols.
Non-interactive modes now reject plan-aware startup, prompt, navigation, fork, clone, new-session, switch-session, and rebind targets before those operations can mutate the active session.

Reasoning

Submitted plans are durable user-facing artifacts and should remain readable, searchable, and copyable in a human session export.
Collaboration-mode changes and resolution records are internal bookkeeping, so the default export view can hide them without hiding the plan that a reviewer actually needs to inspect.

Machine-oriented hosts do not yet implement plan decisions or accepted-plan profile selection.
Suppressing lifecycle events alone would leave an unsafe half-supported state, so print, JSON, and RPC must also fail before entering or adopting plan state and must repeat that check around every asynchronous prompt and replacement boundary.

Changes

• Indexes plan titles and Markdown for HTML search, renders the title in the session tree, and displays the submitted Markdown through the existing sanitized renderer in the transcript.
• Classifies collaboration-mode and plan-resolution records as settings entries so default and no-tools export filters hide bookkeeping while the all-entries view can still identify mode and resolution transitions.
• Narrows the JSON event type to exclude collaboration-mode, plan-submitted, and plan-resolved events, and makes event conversion return no wire value for those internal lifecycle notifications.
• Updates JSON print and RPC subscribers to skip filtered lifecycle events without emitting undefined, while preserving all supported agent and profile-switch events.
• Guards print and JSON mode at startup, on session rebind, before and after each queued prompt, and after any prompt that might have entered plan state so later queued messages cannot run across an unsupported transition.
• Passes plan-aware target validators through extension-driven new-session, switch-session, fork, and tree-navigation actions, validating the selected leaf before the current tree or bound session changes.
• Guards RPC before taking over standard I/O, before every prompt, after session rebind, and across direct new-session, switch-session, fork, clone, and extension command paths.
• Returns normal RPC command failures for rejected plan targets while keeping the current session and leaf intact, including malformed lifecycle targets and accepted continuations restored from another branch.
• Adds focused HTML, print, JSON, and RPC tests for artifact rendering, hidden bookkeeping, startup rejection, target validation, queued-prompt transitions, protocol type exclusion, and streaming steer or follow-up delivery.

Testing

• Validation ran on the final rebased stack: npm run check passed.
• Validation ran on the final rebased stack: the focused Vitest run across 28 affected files passed all 451 tests.
• This intermediate branch was reconstructed as part of the stack and was not checked independently after reconstruction.