Allow imports of system C modules (e.g. SQLite3) in AGENTS.md

[buggerman]

Summary

• Tighten the Pure Swift rule so it actually says what it means: the ban is on third-party C source, bridging headers, and .m files. Importing system modules that ship with the macOS SDK (SQLite3, os.log, Network, UserNotifications, …) is fine — those are how Swift exposes the OS, not "C interop" in the pejorative sense.
• Required cover for the upcoming FTS5-powered scrollback search work, which uses import SQLite3. Without this clarification a reviewer could reasonably flag that PR as a violation; with it, the policy is explicit.
• Adds the wrapping rule that protects the rest of the codebase from the C-API ergonomics: any UnsafeMutablePointer<OpaquePointer?> / sqlite3_* call must live inside a small actor and not appear in any public API.

Changes

• AGENTS.md — two consistent edits:
  • Architecture invariants → Pure Swift bullet rewritten to define "C interop" precisely, allow system-module imports, and require actor-wrapped unsafe surfaces. @objc rules unchanged.
  • What not to do bullet updated to match the carveout (third-party C still banned; system modules fine).

Test plan

• N/A, docs only.

Risk / rollback

• Pure documentation; no behavior change. CLAUDE.md is a symlink to AGENTS.md, so both surfaces update in one commit.
• Revert: this single commit.