feat: add mode authority markers, plan-decline tracking, and improved mode-switching UX

[shayne-snap]

Summary

This PR improves mode management by adding authoritative mode markers, plan-decline tracking, and clearer mode-switching instructions throughout the system.

Changes

Mode Authority & Markers

• Mode authority block — system prompt now explicitly declares the current mode and states to treat any conflicting conversation history, hidden markers, or assistant reasoning as stale
• Mode-changed hidden markers — when mode is switched via SetMode, a hidden message is recorded documenting the transition, including a note that mode changes do not retroactively approve plans
• Plan-decline tracking — a new IntentDeclinePlan intent and RecordPlanNotApproved method record a hidden <plan_not_approved> marker, preventing the agent from acting on a declined plan that remains in history

Mode-Switching UX

• Updated blocked-tool messages in ask mode and plan mode to suggest specific commands (/agent, /plan, /ask) and the Shift+Tab shortcut instead of vague instructions
• Added mode-switching command documentation in the system prompt: Mode switching commands are /agent, /ask, and /plan. Shift+Tab cycles modes in the TUI. Do not tell users to run /mode agent, /mode ask, or /mode plan; those commands do not exist.

Cleanup

• Simplified buildImplementPlanPrompt to drop the redundant plan text (the plan is already in the agent's context window)

Test Coverage

• TestImmutableSystemBlocksDeclareCurrentModeAuthoritatively — verifies the mode authority block and mode-switching instructions appear in system prompts
• TestDeclinePlanPersistsHiddenMarkerAndStaysInPlanMode — tests the plan-decline flow via service dispatch
• TestEscDeclinePlan — tests the TUI popup escape key declines plan and clears stale plan state
• Updated existing test fixtures to match new mode-switching message text

Files Changed

• internal/agent/stream.go — updated mode-blocked messages with specific commands
• internal/agent/system_prompt.go — added mode authority block, mode-switching instructions, date/time instruction
• internal/agent/system_prompt_test.go — new test for mode authority block
• internal/app/app.go — calls RecordModeChanged on mode switch
• internal/app/plan_markers.go — new file with RecordPlanNotApproved and RecordModeChanged
• internal/app/service/dispatch.go — handles IntentDeclinePlan, simplified prompt builder
• internal/app/service/events_test.go — new test for plan-decline dispatch
• internal/tui/chat_view.go — minor cleanup
• internal/tui/model_keys.go — plan-decline key handling with state clearing
• internal/tui/model_test.go — new test for esc decline in TUI