feat(native): add structured notification protocol

[777genius]

Literal --help in notification content currently selects CLI help and exits successfully without sending. Parse options by position and introduce a versioned native request/receipt protocol with strict validation, per-attempt file ownership, and side-effect-free capability discovery.

This is the first bounded checkpoint of agent-initiated notifications. Existing hooks and legacy click actions retain their behavior. The new protocol advertises only none; installer activation, Codex routing, the journal and MCP tools follow in dependent PRs.

Validation:

• 48 macOS Swift package tests passed before the Unicode review fix; all 11 native protocol tests passed after that fix.
• Go nativeprotocol tests passed, including shared Swift wire fixtures.
• Standalone native subprocess smoke passed, including literal flags, correlated rejection receipts, duplicate launches, symlink rejection and joined Unicode; no notifications sent.
• Independent Astra review found one Unicode validation issue, fixed with byte-preserving emoji/ZWNJ regressions. Full installed Desktop E2E and upgrade/cold-start qualification remain later gates.

[coderabbitai]

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