feat: add --manifest flag to pack command

[bryan-anthropic]

Summary

• Adds -m, --manifest <path> option to mcpb pack so users can specify a manifest file outside the project directory
• Manifest resolves relative to CWD, is validated, and injected into the bundle
• Errors immediately with "Manifest file not found" when the explicit path doesn't exist (no interactive prompt)
• Backward compatible — cleanMcpb() and other callers work unchanged

Closes #87

Test plan

• yarn build — clean
• yarn lint — clean
• yarn test — 220 pass (218 existing + 2 new)
• E2E: pack with --manifest → unpack → MCP initialize + tools/list → responds correctly
• E2E: --manifest /nonexistent → exits 1

🤖 Generated with Claude Code

[bryan-anthropic]

Review by Claude Code

Small, clean feature addition. Observations:

• Commander integration is clean — .option('-m, --manifest <path>', ...) follows the existing CLI patterns in cli.ts.
• resolve(customManifestPath) ensures the manifest path resolves relative to CWD, which is the expected behavior for a CLI flag (as opposed to relative to the project directory).
• Manifest injection via files["manifest.json"] = { data, mode } correctly overwrites any manifest that may exist in the project directory. This is intentional — an explicit --manifest flag should take precedence over auto-discovered manifests.
• Error handling is appropriate: when an explicit path is provided but not found, the command errors immediately rather than falling back to interactive prompts. This is the right UX for a flag-driven workflow.
• Both test cases (valid manifest path succeeds, invalid path fails) follow the existing test patterns and cover the key scenarios.
• Fully backward compatible — omitting --manifest preserves existing behavior.

No issues found.