chore(release): add core v0.5.0 changeset

[lIang70]

Adds .release/core-v0.5.0.json: a minor changeset for core.

core: 0.4.11 -> 0.5.0 (minor), tag core/v0.5.0

Why minor

#603 recorded the decision: its commit message and the review disposition both
state that when core is tagged, the changeset must be minor and must name
the tool_search ranking change, not just "shared BM25 kernel". This tag
carries that change — tool_search ranks with prefix expansion, and the
model-visible ranking and hit set change — which is the behaviour change the
repository reserves a pre-1.0 minor for. The delta removes or renames no
exported identifier, so nothing else forces the bump either way;
backends/memory stays untagged.

What the summary covers, since core/v0.4.11

• feat(core): the first upstream batch — backends/secret keychain store
  (non-release-managed), core/tool/middleware Truncate, the
  core/inference/inferencetest scripted wire provider, core/utils
  fsatomic + pathsafe + httpprobe, core/workspace Capped reads, plus the
  review follow-ups.
• feat(core/sandbox): the escalation front end — refusal detection, the
  rules store, the gate, one prompt, one approved retry through the
  unconfined chain (fail closed).
• feat(core/media): the media payload layer — filetype classification,
  imageutil normalization, the media.attachments prepare hook, and their
  budgets.
• feat(core): the shared BM25 kernel and the tool_search ranking change —
  core/utils/bm25, the golden score table, and the catalog contract tests.
• feat(core/skill): the read-only Agent Skills engine — parse, discover,
  service, rank, render.
• fix(core/inference): response validation checks named in Error.Detail
  for unary and terminal streaming (fix(core/inference): name response validation checks #588).

Verification

• make ci, make lint (9 modules, 0 issues), make release-check,
  make release-plan (core: 0.4.11 -> 0.5.0 (minor), tag core/v0.5.0),
  make release-preflight (core: preflight tidy OK), git diff --check.

Next

The Release workflow aggregates this changeset into CHANGELOG.md and opens
the release PR; merging that publishes core/v0.5.0.