feat: add pty_snapshot and pty_snapshot_wait tools with seq-based diffing

[JosXa]

Summary

Adds pty_snapshot and pty_snapshot_wait tools that capture the parsed visible terminal screen of any PTY session, with seq-based history and line-level diffing. Complements the existing pty_read tool which returns the raw output buffer (ANSI codes and all).

Motivation

When using pty_read on TUI applications like OpenCode itself, the output is an unreadable flood of ANSI escape sequences, cursor movements, and control characters. This makes it impossible for AI agents to understand what is actually displayed on screen.

pty_snapshot solves this by maintaining a headless terminal emulator (@xterm/headless) alongside each PTY session, producing clean screen captures that look exactly like what a human would see.

What is included

pty_snapshot tool

• Returns visible screen text, cursor position, terminal size, content hash, and a monotonic seq number
• Optional since parameter: pass a previous seq to get only changed lines (line-level diff) instead of the full screen
• Seq only increments on actual content changes (hash-based dedup)

pty_snapshot_wait tool

• Blocks until a condition is met, then returns the matching snapshot
• search: regex pattern against screen text, resolves on first match
• hashStableMs: resolves when screen content is unchanged for N milliseconds
• timeout: max wait (default 30s)
• Also supports since for diff output

Seq-based history

• Ring buffer of 200 deduped snapshots per session
• Each frame stores line-by-line content for efficient diffing
• Diff format shows changed/added/removed lines with line numbers
• If requested seq fell off the buffer, returns full screen with historyTruncated flag

Other improvements

• Headless terminal emulator per PTY session feeding both raw ring buffer and xterm.js
• Improved web UI plain-text buffer endpoint uses parsed screen instead of naive ANSI stripping
• FNV-1a content hash for efficient change detection without full-text comparison

Example: full snapshot

<pty_snapshot id="pty_abc" seq="5" hash="123...">
Size: 120x40
Cursor: (20, 50) visible=true
---
                         OpenCode banner and clean screen text...

Example: diff since seq 3

<pty_snapshot id="pty_abc" seq="5" hash="456..." since="3">
Changed lines:
  5 [changed]: pineapple pineapple pineapple pineapple pineapple
  7 [changed]: Build gpt-5.4 2.0s

Implementation details

• Uses @xterm/headless (same terminal emulation as VS Code terminal)
• Writes queued with proper callback handling for xterm async time-sliced parsing
• Content hash uses FNV-1a 64-bit for fast, collision-resistant change detection
• Synchronous cached snapshot state avoids async API churn across manager/tools/web layers
• waitForCondition polls at 100ms intervals, checks search regex and/or hash stability