feat(tui): add prompt transform coordinator

[pescao0]

Issue for this PR

Part of #38962

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds the internal host-owned coordinator for composable prompt transformations. It runs registered transforms in deterministic order, serializes work per draft, coalesces identical active or queued requests, rejects stale state, and performs one synchronous host commit.

This is the first of two focused PRs. It intentionally exposes no plugin API yet.

#38962
└── 📍 prompt transform coordinator core
    └── public plugin API and TUI host integration (follow-up)

How did you verify your code works?

• bun test test/prompt/transform.test.ts: 6 passed, 0 failed, 20 assertions.
• Strict standalone TypeScript check for src/prompt/transform.ts: passed.
• Prettier and whitespace checks: passed.

Screenshots / recordings

Not applicable. This slice has no user-visible UI.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR