feat(experimental): add reveal_hidden_cursor_for_cjk_ime opt-in for CJK IME tracking

When the focused pane requests ?25l (cursor hidden) but paints its own
cursor — common in TUIs like Claude Code, pi, and codex — macOS native
input methods stop tracking the candidate-window position. CJK users
(Chinese / Japanese / Korean) cannot see candidate windows follow as
they type. Setting [experimental] reveal_hidden_cursor_for_cjk_ime = true
exposes the focused pane's cursor anchor to the outer terminal regardless
of the pane's hidden-cursor request, restoring IME tracking for CJK input.

[experimental] cjk_ime_agents is an optional allow-list. When empty,
the reveal applies to any focused pane. When non-empty (e.g.
["claude", "codex"]), the reveal only applies if the focused pane's
detected agent matches one of the listed names. This lets users enable
the workaround only for AI-agent TUIs that paint their own cursor,
leaving plain shells and other programs untouched. Accepted names: pi,
claude, codex, gemini, cursor, cline, opencode, copilot, kimi, kiro,
droid, amp, grok, hermes. Unknown names are ignored.

When the pane reports no cursor position (cursor_state returns None for
some TUIs), the anchor falls back to the pane's top-left so a stable
IME hint is always available.

[experimental] cjk_ime_cursor_shape (default "steady_block") controls
the DECSCUSR shape rendered for the revealed cursor; six variants are
accepted: block, steady_block, underline, steady_underline, bar,
steady_bar.

Trade-off when enabled: a hardware cursor will be visible in the outer
terminal for apps that hide the cursor without painting a replacement.
Default false preserves the behavior set in 2c95071. Pair the reveal
with cjk_ime_agents to scope it to specific TUIs.

Manually verified on macOS 26.5 aarch64 with system Pinyin IME against
Claude Code: candidate window tracks cursor with flag enabled, returns
to anchored-at-start behavior with flag disabled.

refs #149

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