fix(app): preserve prompt input editing

[Hona]

Summary

• preserve populated drafts when command-menu built-ins execute
• insert fallback pasted text without flattening structured mentions
• detect @ completion against text before the current cursor

Tests

• bun test src/v2/components/prompt-input (packages/session-ui)
• bun typecheck (packages/session-ui)
• bun typecheck (packages/app)
• bun run typecheck:e2e (packages/app)
• bunx playwright test e2e/regression/prompt-input-v2-editing.spec.ts (packages/app)

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[opencode-agent]

Split into #37942 (command-menu drafts), #37940 (structured mentions during paste), and #37941 (@ completion at the cursor), as requested by @Brendonovich.