Filter terminal reply traffic around tmux attach and detach

[johnrichardrinehart]

The attached-session path can leak terminal reply bytes back into Agent Deck's
Bubble Tea UI during rapid attach/detach cycles. In practice this showed up as
literal fragments such as (0.44.0)/d3d3/f5f5/d3d3/f5f5(0.44.0)... appearing
in the TUI after repeating:

• attach to a tmux-backed session
• detach with Ctrl+Q
• reattach and press Enter a few times

Relevant environment from the repro:

• Agent Deck running inside tmux
• Bubble Tea UI on the host side
• terminal-emulator replies arriving on stdin during redraw / teardown
• the visible reply fragments included terminal version and color-report text

The underlying problem is that terminal protocol replies are not user input, but
we were still letting them cross the attach/detach boundary. The fix here keeps
that handling bounded to those lifecycle edges and treats the problem from first
principles:

• add a small stateful terminal-reply filter that understands protocol classes
  instead of matching terminal-specific payload strings
• discard escape-string replies generically (OSC, DCS, APC, PM, SOS)
• discard non-keyboard CSI replies during the short quarantine window
• preserve normal keyboard input, including CSI/SS3 key sequences such as
  arrows, Home/End, Backtab, and CSI-u key events
• reuse the same filter in both the tmux attach path and the keyboard
  compatibility reader so split replies are handled consistently across reads

This intentionally avoids keying behavior to a specific terminal emulator,
terminal version, or tmux version. The parser is based on control-sequence
structure, not on reply payload contents.

Constraint: normal keyboard input must survive immediately after attach/detach,
including CSI/SS3-based keys

Rejected alternatives:

• match terminal-specific payload strings
  • too brittle against emulator or multiplexer version changes
• drop all CSI traffic during the quarantine window
  • would break legitimate keyboard input
• keep extending PTY drain timeouts alone
  • the failing bytes were arriving on stdin as protocol replies, not just on
    PTY output

Tested:

• nix shell nixpkgs#go nixpkgs#gcc -c go test ./internal/tmux ./internal/ui ./internal/termreply -run 'TestCleanupAttach_|TestCSIuReader(DropsTerminalRepliesDuringQuarantine|DropsSplitTerminalRepliesDuringQuarantine|PreservesNormalInputDuringQuarantine)|TestFilter'

Not tested:

• a fresh packaged build exercised manually through repeated Ctrl+Q / Enter
  cycles after this exact commit