Align obsidian-vault frontmatter schema with obsidian-master-kit

[fabiusilva]

Summary

• mcp-obsidian-vault wrote frontmatter with a bare date field and empty-string defaults (type: "", status: ""), which doesn't match obsidian-master-kit's canonical schema (created/updated/area, status defaulting to "draft"). If a vault is shared between growthOS and obsidian-master-kit, every note growthOS creates gets flagged as a schema mismatch by the librarian skill.
• Switches _FRONTMATTER_DEFAULTS to created+updated (both set on create, updated refreshed on every update()), defaults area to "ai-memory" (the area obsidian-master-kit reserves for AI-generated content), and defaults status to "draft".
• Adds an optional area param to the create_note MCP tool so callers can target pessoal/profissional/pesquisa instead of the default.
• created is now treated as immutable on update() (mirrors the rule obsidian-master-kit's librarian already enforces: a skill that tries to change created gets reverted).
• Updated the 3 test_vault_ops.py assertions that depended on the old date field / empty-string status default.

Test plan

• Manually traced all call sites of vault_ops/VaultOperations in the repo — confirmed nothing else programmatically depends on the date field (only this package's own tests).
• Verified the new create/update/immutable-created behavior with a standalone script exercising VaultOperations directly (sandbox had no pip available to run pytest with real deps installed).
• Please run pytest mcp-servers/mcp-obsidian-vault/tests/ in an environment with python-frontmatter/fastmcp installed to confirm before merging.

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