feat(tui): role-colored gutter, delete-from-index, escape stripping, richer headers

[jugrajsingh]

Summary

Four TUI polish items, bundled as one review pass over the same rendering surface:

1. Role-colored selection gutter. The previous DarkGray background fill on the selected message force-painted every span's background, clobbering per-span foreground styling. Replaced with a leading ▌ gutter in the message's role color (two-space lead on unselected messages keeps columns aligned), so span colors survive selection intact. Important groundwork ahead of markdown/syntect rendering, where span styling carries more information. Body wrap width drops by GUTTER_WIDTH in both the renderer and the viewing-pane row math so scroll offsets stay consistent with what's drawn.
2. Ctrl+X delete-from-index, with a confirm popup. Available in Search (list focused) or Viewing. Confirming calls Store::delete_session_data to remove the index row (message_vec + sessions, cascading); the on-disk source file is untouched and will re-index on next sync unless the source is excluded. The confirm popup wording says exactly that — "removed from index, file kept on disk, will resync" — rather than implying deletion of the underlying file. Cancelling restores the invoking mode.
3. Full ANSI/OSC/C1 escape stripping, security-adjacent. Replaces the previous naive control-char filter with a state machine that walks each escape sequence to its proper terminator: CSI to its final byte (0x40..=0x7e), OSC/DCS/SOS/PM/APC strings to BEL or ST. Covers both the 7-bit ESC-prefixed introducer classes and their 8-bit C1 equivalents (0x90/0x98/0x9b/0x9d/0x9e/0x9f), since some sources emit raw C1 bytes without an ESC prefix. An unterminated sequence at EOF drops the rest of the input rather than leaking partial escape bytes into rendered text. Wired into both the copy path and the render path, so neither pasted nor rendered content can smuggle terminal control sequences into the user's terminal.
4. Source-aware role labels and richer viewing header. The viewing header and message role labels now reflect the originating adapter (e.g. distinguishing a Claude Code sub-agent turn from a top-level one) instead of a flat Assistant/User label, reconciled with the session's actual source metadata.

Verification

• cargo build, cargo fmt -- --check, cargo test --workspace — green.
• 28 new tests covering: gutter color preserved under selection with per-span foreground intact, escape-sequence stripping across CSI/OSC/C1 variants including truncated/unterminated sequences at EOF, delete confirm/cancel state transitions, and role-label resolution across adapter sources.

Test plan

• Select a message with mixed-color spans → span colors survive, gutter shows role color
• Ctrl+X in Search → confirm popup → session removed from index; file on disk untouched
• Ctrl+X → cancel → returns to invoking mode, nothing deleted
• Content with embedded CSI/OSC/C1 sequences → stripped from both render and copy output
• Truncated escape sequence at EOF → dropped cleanly, no leaked control bytes
• Viewing header for a sub-agent turn → shows source-specific label, not generic Assistant