chore(release): v0.15.0

[ajianaz]

What

Release prep: bump cora-code 0.14.0 → 0.15.0 and stamp the CHANGELOG.

Why

Version bump per release checklist (CHANGELOG entry is a tag-blocker). Content of 0.15.0 since v0.14.0:

• feat(brain): opt-in vecq vector store (feat(brain): opt-in vecq vector store backend #543) with keyed persistence, residual default width, and the brain.vector_bits knob (feat(brain): vecq-core 0.3.0 — keyed persistence, residual default, brain.vector_bits knob #548 / chore(deps): bump vecq-core to 0.3.0 #547)
• fix(brain): vector signal dead in fresh processes (fix(brain): load vector index from disk on the search path #546 / brain search: vector signal never fires in a fresh process (VECTOR_CACHE never loaded from disk) #545); stale embed fingerprints after a global index rebuild
• docs(cli): document cora upgrade (docs(cli): document cora upgrade — self-upgrade was undocumented #549 / docs: document cora upgrade in cli-reference (self-upgrade is undocumented) #541)

Testing

Full suite green on develop; CI validates this PR.

Release steps after merge

1. develop → main via release/v0.15.0 branch PR
2. Tag v0.15.0 on main → release.yml publishes