feat: add Flutter integration pack

[ElbertePlinio]

Summary

• add the default-off pickforge-flutter integration pack behind hidden --mobile-integration-alpha
• configure the official Dart/Flutter MCP command for Claude Code, Codex, and Pi
• install one embedded portable Flutter workflow outside the project tree
• preserve PR 2's dry-run, backups, rollback, idempotency, and schema-v1 receipt contracts

Behavior

The alpha pack registers Pickforge-owned pickforge-dart:

• Claude Code: dart mcp-server
• Codex: dart mcp-server --force-roots-fallback
• Pi: dart mcp-server --force-roots-fallback (via pi-mcp-adapter)

The workflow is installed transactionally to:

• ~/.claude/skills/pickforge-flutter/SKILL.md for Claude Code
• ~/.agents/skills/pickforge-flutter/SKILL.md for Codex and Pi

Normal pickforge init still selects the empty base pack. This PR adds no packaging or release exposure.

Safety

• planning never writes, executes dart, starts subprocesses, or uses the network
• missing dart fails before target inspection or writes
• project files remain untouched
• foreign workflow files fail closed
• Pickforge-owned workflow updates receive backups
• physical aliases between skill roots deduplicate safely
• config, workflow, and receipt writes share one transactional apply/rollback

Validation

• cargo fmt --all --check
• cargo clippy --workspace --all-targets --locked -- -D warnings
• cargo test --workspace --locked — 65 tests
• Windows MSVC all-target cargo check and cargo clippy -D warnings
• Bun frozen install, typecheck, lint, and build
• OSV Scanner 2.3.8 — no unfiltered advisories
• gitleaks 8.30.1 — no leaks in branch history
• isolated all-harness dry-run/apply smoke — six actions, successful apply, clean project Git tree
• serious-backend local review and targeted fix verification — clean

Not tested locally

• Windows-native execution (required PR CI job)
• headed browser/desktop TypeScript integration path (local Xvfb is absent; required PR CI installs it)
• macOS

Refs #64
Depends on #68

Merge authorization: Elberte only.