feat: add validate command for note quality checking

[leobrival]

Summary

Add a new validate command that checks Obsidian vault notes for common quality issues.

Usage

# Validate entire vault
notesmd-cli validate --vault MyVault

# Validate a single note
notesmd-cli validate daily/2024-01-15 --vault MyVault

# JSON output
notesmd-cli validate --vault MyVault -f json

# Errors only
notesmd-cli validate --vault MyVault -s error

Rules implemented

Rule ID	Severity	Description
frontmatter.parse-error	error	Invalid YAML frontmatter
frontmatter.missing	warning	Note without frontmatter
frontmatter.deprecated-singular	warning	tag/alias/cssclass should be tags/aliases/cssclasses
frontmatter.list-type	error	tags/aliases/cssclasses must be lists
frontmatter.tags-hash	warning	Tags with # prefix in YAML
structure.empty-note	warning	Empty or whitespace-only note
structure.heading-hierarchy	warning	Skipped heading levels (e.g. H1 to H3)

Flags

Flag	Short	Default	Description
--vault	-v	""	Vault name
--format	-f	text	Output: text, json, summary
--severity	-s	warning	Minimum severity: error, warning, info

Architecture

• cmd/validate.go — Cobra command (registered via init())
• pkg/validate/result.go — Types (ValidationResult, Severity, Summary)
• pkg/validate/config.go — Default config and directory exclusions
• pkg/validate/validate.go — Orchestrator (walk vault, collect results)
• pkg/validate/rules/frontmatter.go — 5 frontmatter rules
• pkg/validate/rules/structure.go — 2 structure rules

Reuses existing patterns: VaultManager interface, frontmatter.Parse(), filepath.WalkDir, mock pattern from mocks/vault.go.

Exit code 1 when errors are found (useful for CI).

Checklist

• I have written unit tests for my changes
• I have updated the documentation accordingly
• All tests pass (go test ./...)

[Yakitrak]

Hey thanks for the PR! It is well-written code, but it feels like it belongs in a dedicated vault-linting tool rather than here. If someone wanted serious vault validation, they'd want more rules, configurable rule sets, ignore files, etc., then scope creep would start pulling the project in a different direction. If this gets requested more then I can revisit but otherwise I don't think its the best fit here. Please feel free to contribute on anything else on the issues list if you want to.