Remove Codex slash command compatibility layer

[laotoutou]

What

• Remove the browser Codex slash-command catalog, parser, completion menu, Plan-mode banner, and command-only dialogs.
• Submit leading / text as ordinary visible user input from both New Session and active Session composers.
• Remove the slash-only native RPC/service and public API projections for review, compaction, goals, personality, token usage, hooks, Session memories, background terminals, and Agent Threads.
• Keep explicit browser controls such as Session/Turn fork, rename/archive, model/reasoning/Fast selection, file mentions, Workspace, Skills, MCP, network, and credentials.
• Keep Environment memory APIs because the Sandpi CLI consumes them independently, and keep historical native review/compaction projection so existing Sessions remain readable.
• Regenerate the OpenAPI contract and update architecture docs and tests.

Why

Codex app-server does not expose the TUI slash catalog as a runtime capability. Sandpi was therefore mirroring TUI commands and their native protocol details by hand, creating unnecessary coupling to the pinned Codex version and a large drift-prone compatibility layer for operations the browser already represents directly.

Impact

• The composer no longer opens a slash menu or intercepts slash-shaped text; for example, /plan ... is sent verbatim to Codex.
• The OpenAPI surface drops 19 command-only operations, from 121 to 102 operations.
• Automation-owned collaborationMode remains supported for Schedules and Webhooks.
• Existing explicit UI flows and durable native history rendering remain supported.

Checks

• npm run lint
• npm run typecheck
• npm test (693 tests)
• GOTOOLCHAIN=go1.22.0 npm run test:cli
• npm run openapi:check
• npm run build
• npx playwright test e2e/smoke.spec.ts --grep "deduplicates native models and submits leading slash text verbatim" --workers=1
• Live preview and bootstrap returned HTTP 200 on http://172.16.100.2:3000