fix: make coding agents standalone singleton pages

[dhhieu113pro]

Goal

Treat Copilot, Codex, and Antigravity as dedicated singleton Dev pages rather than terminal sessions.

Expected behavior:

• each built-in agent has one dedicated session per workspace
• built-in agents do not enter the Sessions collection
• built-in agents do not appear in the terminal tab strip
• built-in agents do not affect Terminals (N)
• built-in agent titles stay exactly Copilot, Codex, and Antigravity (no terminal numbering)
• regular terminals continue to own the terminal count/tab collection

Implementation

• added dedicated Copilot, Codex, and Antigravity DevSpace pages
• built-in agents are cached as singleton sessions outside the regular terminal Sessions collection
• added a dedicated agent surface grid, separate from terminal layout/tabs
• sidebar agent navigation now routes directly to the dedicated agent page
• regular terminal profiles and terminal layout behavior remain unchanged

TDD / verification

• RED: the new regression contract failed on the original terminal-coupled behavior (CI run 392 / commit 088edaf)
• GREEN: focused regression tests and DevSpace dashboard regression tests pass on current head 8bdebd4 (Continuous Integration run 398)
• all build/package jobs in Continuous Integration run 398 pass
• DevSpaces Screenshots run 570 passes and publishes/launches the real Windows app
• Store MSIX run 394 passes, including Store contract/unit tests and x64/arm64 packaging