mecatui: allow selecting and copying text from read-only session transcripts

Problem

Read-only session inspection prevents users from copying conversation content into another chat or application. Read-only should prohibit changing the session, not extracting its text.

Reported while using /sessions in the ToolHive workspace: one session showed a running triangle instead of a completed checkmark and could only be inspected, not continued. The user then could not select/copy its content to paste elsewhere. This removes a practical escape hatch when session continuation is unavailable.

The session's actual runtime ownership and reason for remaining running have not been verified. Recovery is a separate concern; copying an inspectable transcript should work regardless of whether that session can be continued.

Reproduction

1. Open /sessions in mecatui with the default mouse-enabled UI.
2. Select a session whose capabilities permit inspection but not public-chat continuation (for example, a session reported as running elsewhere).
3. Open its read-only transcript.
4. Attempt to select and copy conversation text for pasting into another chat or application.

Actual: the transcript can be scrolled, but there is no in-app transcript text selection/copy action. The session list's y: copy ID copies only the session ID.

Expected: an inspectable transcript remains easy to select and copy, even when continuation is disabled.

Source findings

• Read-only inspection is opened in cmd/mecatui/ui/sessions_surface.go:569-580; its transcript viewport and keyboard handling are at lines 401-443.
• Main-conversation selection rejects open modals in cmd/mecatui/ui/selection.go:105-131; the sessions inspector is a modal, and mouse handling consumes modal clicks before conversation selection (cmd/mecatui/ui/update.go:3937-3991).
• Existing main-chat selection uses OSC52 plus a clipboard fallback (cmd/mecatui/ui/update.go:4301-4310), but this is not wired into the inspector.
• y: copy ID is a separate action (cmd/mecatui/ui/sessions_surface.go:500-515).
• Paste is blocked while a modal is open (cmd/mecatui/ui/update.go:2621-2633). Pasting into a read-only transcript is not required; the desired workflow is copying out and pasting elsewhere.

Line references describe the source inspected when filing and may move.

Acceptance criteria

• Users can select and copy text from read-only /sessions transcripts without reopening them as writable chats or restarting with different terminal flags.
• A discoverable keyboard-accessible copy action is available; help text distinguishes transcript copying from session-ID copying.
• Copied content is usable plain text, preserves relevant line breaks, and excludes UI decorations and ANSI control sequences.
• Existing clipboard mechanisms are reused where suitable, with honest success/failure feedback rather than silent loss.
• Copying never starts, resumes, cancels, or otherwise mutates the inspected session, and does not weaken transcript authorization.
• Scrolling and closing the inspector continue to work, including for long transcripts.
• Offline regression tests cover the read-only inspector path, not only main-chat selection.

Workaround / scope

Source documents --no-mouse or inline/no-alternate-screen operation as allowing terminal-native selection. Requiring a restart or terminal-mode change is not an adequate normal workflow, and terminal behavior may vary.

Keep this issue focused on copying from read-only inspection. Fixing stale/running session recovery, adding editable inspection, and broader transcript export formats are separate work.

Start at the read-only inspector in cmd/mecatui/ui/sessions_surface.go, especially the transcript viewport and keyboard handling, then trace modal selection and mouse handling in selection.go and update.go. Compare the existing OSC52 and clipboard fallback path, add a discoverable transcript-copy flow with plain-text feedback, and cover the read-only inspector with offline regression tests while preserving scrolling, closing, and session immutability.