feat(desktop): add /copy session transcript command

[anduimagui]

What does this PR do?

Desktop did not support the /copy session transcript command that exists in TUI, so users could not copy full transcripts from the desktop command flow.

This PR adds desktop parity by introducing session.copy in app commands, wiring native desktop clipboard write, and adding the required desktop capability permission. It also moves transcript formatting to @opencode-ai/util so the app imports a shared formatter as a first step toward unifying Desktop/TUI copy behavior in a follow-up.

Type of change

• feat (adds a feature)
• fix (non-breaking bug fix)
• docs (documentation only)
• refactor (non-breaking internal improvement)

How did you verify your code works?

• Ran bun --cwd packages/util run typecheck
• Ran bun --cwd packages/app run build
• Ran bun --cwd packages/desktop tauri dev
• Manually tested in Desktop app: executed /copy in a session and verified transcript content was copied to clipboard
• Rebasing branch onto latest dev resolved prior packages/desktop/src/menu.ts conflict and PR is mergeable

Checklist

• I tested these changes locally
• I only included changes related to this PR
• I updated permissions/config where required (clipboard-manager:allow-write-text)

Screenshots

Issue for this PR

Closes#12545

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File-level summary

• packages/app/src/pages/session/use-session-commands.tsx: add session.copy slash command and use shared transcript formatter
• packages/util/src/session-transcript.ts: shared transcript formatting utility (app now imports this)
• packages/desktop/src/index.tsx: native clipboard write via Tauri clipboard manager
• packages/desktop/src-tauri/capabilities/default.json: add clipboard-manager:allow-write-text
• packages/desktop/src/menu.ts: add menu action for session.copy

Follow-up (separate PR): migrate TUI to this shared formatter path for a single unified copy implementation across TUI/Desktop.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[rekram1-node]

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