docs(readme): rewrite around what the package is

[TheGreatAxios]

Summary

Stacked on #56; merge that first.

• The README opening now says what the package is (semantic and full-text document memory for Interchange, stored in Postgres/pgvector) and when to use it.
• Environment variables move out of a quickstart paragraph into a Reference table (variable, required, default, meaning). Each default was checked against the source.
• The internal terms "plane", "grant tags" and "sidecar bearer token" are replaced with plain wording.
• The resident distiller is introduced once. The @corbits/cron wiring paragraph is now one line: the host decides when to mail mailTo.
• The README Development section moves to CONTRIBUTING.md. The dead bun run dev line is dropped, because src/server.ts does not exist.
• The runMemoryMigrations section says each run replays in one transaction behind an advisory lock, so every replica can call it at boot.
• "Upgrading from 0.1" lists the helpers no longer exported from the package root, checked against npm 0.1.0's exports, and notes runMemoryMigrations now comes only from @corbits/memory/migrations.
• The createResidentDistiller JSDoc example destructures { workflow, agent }, and its text matches the README.

Verification

• bun run typecheck passes.
• Every default in the Reference table was checked against src/mount-config.ts, src/core/embed-client.ts, src/core/rerank-client.ts and src/core/fts-language.ts.

Closes CL-9176
Closes CL-9177
Closes CL-9178
Closes CL-9179
Closes CL-9194
Closes CL-9328
Closes CL-9169

[TheGreatAxios]

Self-review, two passes.

Verdict: ready for review.

• Every README claim was checked against src/.
• Review findings were fixed: the leftover "plane", the distiller wording now matching between README and JSDoc, and CONTRIBUTING keeping the unit-test and Ollama notes.