fix(terminal): prevent macOS dictation from duplicating text in the task terminal

[binghaobhw]

Summary

Ending macOS dictation with the Command key delivers a bare Meta keydown while the dictation composition session is still active (isComposing: true). xterm's CompositionHelper.keydown treats any keydown outside its Shift/Ctrl/Alt/CapsLock/IME(229) exemption list as a real key interrupting the composition and eagerly flushes the composed text to the PTY; the subsequent compositionend then flushes the same text again — so every dictated phrase appeared twice in the terminal.

Captured trace (textarea events + WebSocket sends) showing the double flush:

[15918.6] WS-SEND " 嗯，哈喽哈喽"                        ← eager flush
[15918.9] keydown key="Meta" keyCode=93 isComposing=true  ← Cmd pressed to end dictation
[16166.8] compositionend data=" 嗯，哈喽哈喽"
[16167.4] WS-SEND " 嗯，哈喽哈喽"                        ← second flush, identical

The fix swallows bare Meta presses in attachCustomKeyEventHandler, which runs before xterm's composition handling. A lone Meta press has no meaning to a terminal, and combo shortcuts (Cmd+C / Cmd+V) are unaffected because those events arrive keyed by the non-modifier key.

The underlying defect (Meta missing from the modifier exemptions) will also be reported upstream to xtermjs/xterm.js.

fixes #566

Test plan

• Manually verified on macOS 12 + Chrome against a source build: dictating into the task terminal and ending with Cmd no longer duplicates the phrase; Cmd-based shortcuts still work; CJK IME typing unaffected (its candidate keys arrive as keyCode 229, already exempt)
• npm run check passes (pre-commit hook: typecheck + fast tests)
• No automated test: the behavior lives in the interaction between a real KeyboardEvent stream and xterm's internal CompositionHelper, which the existing test setup does not simulate

🤖 Generated with Claude Code

[greptile-apps]

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