Remove legacy top-level gitignoreEntries from manifest

[bguidolim]

Summary

The top-level gitignoreEntries field on ExternalPackManifest is labeled "Legacy" in docs/techpack-schema.md with guidance to prefer the gitignore: component shorthand instead. Like hookContributions (removed in #156), it's a redundant mechanism that duplicates what the modern component system already provides.

Current state

gitignoreEntries is wired through:

• ExternalPackManifest.swift — decoded from YAML
• ExternalPackAdapter.swift — forwarded to TechPack.gitignoreEntries
• ComponentExecutor.swift — applied during install
• PackUninstaller.swift — cleaned up during removal
• CoreDoctorChecks.swift — checked during doctor
• PackCommand.swift — counted during pack remove preview

The modern replacement is the gitignore: component shorthand:

# Modern (component)
components:
  - id: my-pack.ignore
    description: Gitignore entries
    gitignore:
      - .build
      - .xcodebuildmcp

# Legacy (top-level field)
gitignoreEntries:
  - .build
  - .xcodebuildmcp

Motivation

Same rationale as #156 — the product is pre-1.0, legacy code paths add complexity without proportional value, and the modern component system already handles gitignore entries with better per-project convergence support.

Scope

• Remove gitignoreEntries from TechPack protocol
• Remove field from ExternalPackManifest
• Remove from adapter, executor, uninstaller, doctor, and command UI
• Update docs
• Clean up tests