Add in-session cancelled command and agent prompt history managers

[Copilot]

Users had no way to recall commands they Ctrl+C'd or prompts previously sent to agent mode within a session.

New behavior

• Ctrl+C on non-empty buffer — saves buffer to cancelled_command_history before exiting (existing exit behavior preserved)
• Ctrl+C on empty buffer — opens a fuzzy picker for cancelled commands (same UX as Ctrl+R) if cancelled history exists
• Shift+Enter on empty buffer (AI configured, agent history exists) — opens a fuzzy picker for agent prompts instead of immediately launching AI mode
• start_ai_mode() — pushes current buffer to agent_prompt_history before spawning the AI subprocess

Implementation

src/history.rs

Added HistoryManager::empty() constructor to create lightweight in-session-only managers (no file I/O; entries are lost on session exit). Also added push() and is_empty() methods to HistoryManager.

src/app/mod.rs

• Added FuzzyHistorySource enum (BashHistory, CancelledCommands, AgentPrompts) with a label() helper
• Extended ContentMode::FuzzyHistorySearch to carry a FuzzyHistorySource, replacing the need for separate mode variants — a single content mode handles all three history managers
• Added cancelled_command_history and agent_prompt_history fields on App (typed as HistoryManager, initialized with HistoryManager::empty())
• Added ContentMode::fuzzy_source() and App::history_manager_for(source) helpers to eliminate repeated dispatch matches across key handlers, mouse handlers, and the accept method
• Full key handling (Up/Down/PageUp/PageDown/Enter/Shift+Enter/Tab/Esc) and mouse hover/click via the unified mode
• Extracted the fuzzy history rendering block into a render_fuzzy_history_results() free function shared by all three history sources; moved ts_to_timeago_string_5chars() to module level accordingly

Original prompt

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This section details on the original issue you should resolve

<issue_title>ctrl+c, agent mode history buffers</issue_title>
<issue_description>I often want to go back to the previous prompt I used for agent mode and previous command I ctrl+c'ed.
We could store them for the current session.
Then when you press ctrl+c on an emtpy line or shift enter on an empty, it will act like ctrl+r but for the other history buffers.

You will need to create two other history managers: cancelled _command_history_manager and agent_prompt_history_manager.
Store them in the settings struct.

The app will push new entries to them.
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes ctrl+c, agent mode history buffers #303

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[HalFrgrd]

I only want one content mode to handle all three different history managers. Figure out the best way to make this happen. This will make the code change small

Don't create a new history session manager class. Reuse the current history manager. Give it a constructor called empty to generate empty ones.

[Copilot]

I only want one content mode to handle all three different history managers. Figure out the best way to make this happen. This will make the code change small

Don't create a new history session mana...

Done in fd83cbc.

• SessionHistoryManager is gone — HistoryManager::empty() creates a lightweight in-session manager using the same struct (plus new push() and is_empty() methods).
• The three content mode variants are now one: ContentMode::FuzzyHistorySearch(FuzzyHistorySource) where FuzzyHistorySource is a Copy enum (BashHistory, CancelledCommands, AgentPrompts) with a label() helper.
• Added ContentMode::fuzzy_source() and App::history_manager_for(source) helpers to remove the repeated dispatch patterns from key handlers, mouse handlers, and the accept method.