feat: Add OSC 9 notification support for TUI

[linw1995]

Closes #135

Summary

Implements OSC 9 escape sequence support for desktop notifications in the TUI, addressing the need for user notification when:

• Subagent requires user confirmation/feedback
• Long-running tasks complete
• Terminal is not in focus

Changes

Core Implementation

• New notifications module (crates/coco-tui/src/notifications.rs):
  • OSC 9 escape sequence backend (default)
  • External command backend support for custom notification handlers
  • Proper payload sanitization and length limits

TUI Integration

• Focus tracking (app.rs, events.rs):

  • Added FocusGained/FocusLost event handling
  • Track terminal focus state to enable "only_when_unfocused" mode

• Chat component (components/chat.rs):

  • Notify when reply is ready (non-tool-use responses)
  • Notify when action is required (tool permissions, text edit confirmations)
  • Configurable notification behavior based on focus state

Configuration

• New config options (src/config/ui.rs):

  [ui.notifications]
  enabled = true              # Enable/disable notifications
  only_when_unfocused = true  # Only notify when terminal loses focus
  backend = { type = "osc9" }  # or external_command

Usage

Default (OSC 9) - works with modern terminal emulators:

[ui.notifications]
enabled = true
only_when_unfocused = true

Custom notification command:

[ui.notifications.backend]
type = "external_command"
executable = "notify-send"
args = ["--app-name", "coco", "--category", "im.received"]

[codecov-commenter]

Codecov Report

❌ Patch coverage is 41.40969% with 133 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.83%. Comparing base (a46f02c) to head (9027a9a).
⚠️ Report is 4 commits behind head on dev.

Files with missing lines	Patch %	Lines
crates/coco-tui/src/components/chat.rs	0.00%	67 Missing ⚠️
crates/coco-tui/src/notifications.rs	49.55%	57 Missing ⚠️
crates/coco-tui/src/app.rs	0.00%	8 Missing ⚠️
src/config.rs	96.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #139      +/-   ##
==========================================
- Coverage   50.22%   48.83%   -1.40%     
==========================================
  Files          93       95       +2     
  Lines       25033    26076    +1043     
==========================================
+ Hits        12574    12734     +160     
- Misses      12459    13342     +883     

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