fix(mcp): advertise precise object-rooted output schemas

[orga-agent]

Problem

mcporter rejects the flicknote MCP server's tools/list because derived output schemas contain terms strict clients cannot parse:

• note_get's metadata field (Option<serde_json::Value>) derives a boolean schema term "metadata": true.
• note_source's SourceResult enum derives a oneOf union without an explicit object root.

Earlier coarse fixes (e2fd1b5, #67; and the first commit here) replaced these with {"type":"object"}, which satisfies strict clients but discards the useful output contract: clients could no longer discover fields, required values, collection shapes, or source-view variants.

Fix

Compatibility is fixed at the narrow incompatible parts instead of erasing each schema:

• note_get: keep the full generated schema; metadata uses an object-form JSON Schema term that keeps arbitrary JSON (and null from Option) representable.
• note_source: new MCP boundary DTO McpSourceResult (transparent over SourceResult, serialization unchanged) advertises an object-rooted tagged union with rendered/raw/info variants, each variant's discriminator and required fields, and an object-form term for the arbitrary raw value.
• List tools: unchanged object wrappers with notes/projects array item schemas.
• Removed the generic minimal-object output_schema override; schemas now follow MCP boundary DTOs.

Tests

• mcp_tool_output_schemas_are_strict_client_compatible: every tool's outputSchema has an explicit object root and no boolean schema terms; list tools advertise wrapper + item schemas.
• mcp_note_get_output_schema_advertises_detail_structure: content/metadata/extractions/sections present and required; metadata is object-form and represents arbitrary JSON incl. null.
• mcp_note_source_output_schema_advertises_all_views: all three views with discriminators and variant-specific fields.
• Existing tool-call tests (note detail, source views, projects) verify structuredContent still matches.

Verification

• cargo test --bin flicknote — 51 tests pass.
• cargo clippy --workspace --all-targets --all-features -- -D warnings clean.
• cargo fmt --all --check clean.
• End-to-end stdio probe of built flicknote mcp: 25 tools, note_get.metadata object-form, note_source object-rooted with views rendered/raw/info, zero strict-client violations.