release: v0.21.0 — tag management

[Fail-Safe]

Release summary

Noema v0.21.0 adds safe cortex-wide tag management to the Rust CLI and MCP server.

Highlights

• inspect tag counts, visibility, tier, and engagement with noema tags stats
• preview and apply bulk rename/delete operations, including opt-in ASCII case-insensitive matching
• diagnose and deterministically repair noncanonical tags with noema tags doctor
• expose equivalent MCP statistics, diagnosis, rename, and delete tools with explicit preview/apply behavior
• preserve long-tier trace bodies, identity fields, content hashes, and timestamps during tag-only mutations
• make remove --force remain recoverable; --force now only overrides source-lock protection
• add recover --force for restoring source-locked traces

Versioning

• bump the Rust package and lockfile to 0.21.0
• report 0.21.0 through MCP server metadata
• update current release/download examples

Validation

• feature PR feat(tags): add cortex-wide tag management #145 CI passed both required jobs
• make test
• make release-check (noema v0.21.0, Rust implementation)
• cd plugins/obsidian && npm ci --silent && npm run build && npx tsc --noEmit
• clean release diff and signed commits verified
• manual CLI validation of tag statistics, doctor plans/fixes, confirmation prompts, and deletion

After this PR is merged and main CI is green, tag the exact merge commit as v0.21.0 to trigger the six-platform release workflow, checksums, plugin archives, and Homebrew publication.