feat(tui v2): selection/paste/scroll/reload polish

[nianyucatfish]

Eight independent fixes accumulated since the last batch, all landing in tuiapp_v2.py:

1. Code-block selection offset: _align_md_renders's is_centered heuristic tripped on any code line indented >=4 spaces (Python's default), stripping the indent from the source string so the visible selection and the copied text disagreed. Add wide_lead >= 2*nt0_lead guard — only Rich's H1 centering (wide_lead ~ width/2) crosses it, code lines never do.

2. Blockquote-continuation selection offset: Rich re-emits ▌ on every wrapped visual line of a quote, but the wide-render carries the marker only once. Strip the marker as a decoration prefix (counts as indent, not content) and switch the wrap-accumulator to account for the per-break eaten space — both pre-existing code-block wraps and blockquotes now map correctly.

3. Streaming ANSI garbage: while a turn is streaming, the tail text segment was rendered with Text(content, style=C_FG) instead of Text.from_ansi. SGR codes from the model/tool output would render as literal [31m glyphs and only resolve after m.done flipped or a resize forced a remount via the Markdown path. Switch the two streaming fast paths to Text.from_ansi — O(n) scan, no Markdown parse, colors honored live.

4. continue picker LIMIT removed: tuiapp's own LIMIT = 20 truncated the /continue picker even though list_sessions() always builds the full list. Show everything; header now reads "选择要恢复的会话 (N 条)".

5. Long-paste rescue + user echo elision: Windows Terminal yanks window focus when its >=5KiB paste-warning dialog pops, leaving focused=None after the user confirms; Textual then routes the Paste event to the App bubble instead of InputArea, and the paste appears to do nothing. Add a GenericAgentTUI._on_paste forwarder that refocuses #input and replays the event into InputArea._on_paste. Separately, on submit, elide user-side display of very long pastes (head 10 / tail 5 / ⋯ 省略 N 行 ⋯); the agent still receives the full content via expand_placeholders.

6. mykey.py hot reload: load_llm_sessions() already had mtime-gated reload plumbing, but it only ran on Agent.init / next_llm / list_llms — so editing mykey.py mid-session required restart. Two triggers added: on_input_area_submitted now calls load_llm_sessions() before sending (cheap mtime check; rebuilds on change), and a new /reload-keys command forces a sweep across every session (resetting llmcore._mykey_mtime per-iteration so the cache doesn't short-circuit later sessions).

7. Resize / sidebar toggle no longer jumps to bottom: _remount_current_session used to unconditionally scroll_end after rebuilding. Capture scroll_y + "was at bottom?" first; stick to bottom only if the user was already there, otherwise restore the prior scroll_y so resizes don't yank the user away from what they were reading.

8. Markdown table cells wrap instead of ellipsizing: Rich's Markdown TableElement passes no overflow to Table.add_column, so the default "ellipsis" hides long cell content with … on narrow widths. Patch TableElement.rich_console on import to pass overflow="fold".