VS 2026: Core services — .ai-team file parsing in C#

## Description
Port the core file parsing services from the TypeScript VS Code extension to C#/.NET. These services form the data layer for the VS 2026 extension, reading and watching the `.ai-team/` directory structure.

This issue depends on Issue #43 (project scaffold) being complete.

## Acceptance Criteria
- [ ] TeamMdService equivalent implemented (parses `team.md`)
- Extracts team members, roles, status, GitHub usernames
- Returns strong-typed TeamMember model
- [ ] DecisionService equivalent implemented (parses `decisions.md` and `decisions/` directory)
- Reads main decisions.md file
- Scans decisions/ subdirectory for decision entries
- Parses H2/H3 headings and date metadata
- Returns DecisionEntry model list
- [ ] SkillCatalogService equivalent implemented
- Scans `skills/` directory
- Parses skill YAML/markdown metadata
- Returns Skill model with name, description, confidence
- [ ] Shared models defined (TeamMember, DecisionEntry, Skill, SkillEntry, etc.)
- Models reflect TypeScript equivalents from `src/models/index.ts`
- [ ] FileWatcherService implemented
- Watches `.ai-team/` directory tree for changes
- Debounces rapid file changes
- Triggers service refresh callbacks
- Clean start/stop lifecycle
- [ ] Service cache invalidation mechanism
- Services can register invalidation callbacks
- File changes trigger appropriate service refreshes
- [ ] All services have unit tests
- Mock file I/O tests
- Parser logic tests
- File watcher debounce tests

## Implementation Details

### Models (C# equivalents)
Create `Models/` folder with:
- `TeamMember.cs` — Name, Role, Status, GitHubUsername, Biography, etc.
- `DecisionEntry.cs` — Title, Date, Authors, Content, Tags
- `Skill.cs` — Name, Description, SourceUrl, ConfidenceLevel, InstallCommand
- `SkillEntry.cs` — lightweight version for catalog listings
- `FileWatcherEvent.cs` — Path, ChangeType, Timestamp

### Services (C# equivalents)
Create `Services/` folder with:
- `TeamMdService.cs` — Parse `team.md` → `List`
- Reference: `src/services/TeamMdService.ts` (TypeScript version)
- Key methods: `GetTeamMembers()`, `GetMemberByUsername()`

- `DecisionService.cs` — Parse decisions → `List`
- Reference: `src/services/DecisionService.ts`
- Key methods: `GetDecisions()`, `GetRecentDecisions(days)`, `GetDecisionsByAgent()`

- `SkillCatalogService.cs` — Parse skills/ directory → `List`
- Reference: `src/services/SkillCatalogService.ts`
- Key methods: `GetSkills()`, `GetSkillByName()`, `SearchSkills(query)`

- `FileWatcherService.cs` — Watch `.ai-team/` for changes
- Reference: `src/services/FileWatcherService.ts`
- Key methods: `Start()`, `Stop()`, `RegisterCacheInvalidator()`, `OnFileChange()`
- Use `FileSystemWatcher` for file monitoring
- Implement debounce (300-500ms) to handle rapid changes

### Data Flow
```
FileWatcherService detects change
↓
Triggers CacheInvalidator callbacks
↓
TeamMdService / DecisionService / SkillCatalogService refresh their caches
↓
Services emit refresh events
↓
UI layer (tool windows, commands) update their views
```

## Size Estimate
**L** (Large) — 8-10 hours

## Priority
**P1** (Critical Path)

## Dependencies
- **Depends on:** Issue #43 (VS 2026: Project scaffold and VSIX configuration)

## Notes
- Focus on accurate file parsing and caching; don't optimize file I/O yet
- Use .NET's `System.IO.FileSystemWatcher` for change detection
- Markdown parsing can be simple regex-based or use a markdown NuGet package (e.g., MarkdownSharp, Markdig)
- Keep models serializable (JSON) for future dashboard/API usage
- Write tests for parsers with fixture files from the actual `.ai-team/` directory
- Assigned to: **squad:virgil** (VS 2026 Extension Dev)