feat!: namespaces, lint mode, and markdown-aware task parsing

[swedishborgie]

Add multi-team isolation via namespaces, dry-run lint validation, and goldmark-based parsing so slash commands inside code blocks are ignored.

Namespaces:

• Support namespace/taskname syntax (e.g. myteam/fix-bug) for shared .agents/
• Tasks, rules, commands, and skills under .agents/namespaces//
• Namespace assets take precedence over global; rules and skills merge
• Add docs/how-to/use-namespaces.md and namespace selector in rule frontmatter

Lint mode:

• Run context assembly without bootstrap or shell execution
• Track loaded files and collect structural errors (parse, missing commands, skill validation, selector mismatches) in LintResult
• Add WithLint() option, ListTasks(), and include_unmatched task frontmatter
• Parse errors include line/column; selector/command errors use task path

Task parser:

• Integrate goldmark Extension for AST-based code range detection
• Slash commands in fenced, indented, and HTML code blocks no longer detected
• ParseMarkdownFile populates Markdown.Task; re-parse only on content change

Infrastructure:

• Enable golangci-lint with comprehensive linter configuration
• Fix lint issues across codebase (revive, gosec, wrapcheck, etc.)
• Add t.Parallel() to tests; expand test coverage
• Documentation corrections and new how-to guides

Still a draft, have to work out a few additional details.