feat(tui): add clipboard copy support to observation screens

[Snakeblack]

Summary

The TUI currently lets users navigate and view observations, but there's no way to copy their content to the clipboard. Users who find a useful memory must manually select text or leave the TUI to retrieve it via CLI.

Proposal

Add a c keybinding that copies the content of the selected/displayed observation to the system clipboard using OSC 52 terminal escape sequences (via go-osc52/v2, already a transitive dependency).

Screens affected

• Recent Observations — copy selected item's content
• Observation Detail — copy the full observation content
• Search Results — copy selected result's content
• Session Detail — copy selected observation's content

Why OSC 52

• Works over SSH sessions (critical for CLI tool users)
• No system dependencies (no xclip/xsel/pbcopy needed)
• Already available as transitive dep from the Charm ecosystem
• Supported by 95%+ of modern terminals (Windows Terminal, VS Code, iTerm2, Kitty, WezTerm)

UX

• Press c → content is copied → "✓ Copied!" feedback appears for 2 seconds
• If the terminal doesn't support OSC 52 → "✗ Copy failed" feedback
• Help text on each screen updated to show the new keybinding

Acceptance Criteria

• c copies the observation content on all 4 screens listed above
• Visual feedback is shown for 2 seconds after copy
• Graceful failure on unsupported terminals
• No new dependencies added (uses existing transitive dep)
• All existing TUI tests continue passing