docs(migration): correct the false "uses: is redirected on rename" claim

[vaderyang]

What

Correct a factually wrong claim in the migration guidance, discovered while
performing the Netis/agent-ops → Netis/olympus GitHub repo rename.

docs/migration.md (and pantheon.md + the migration linter's wording) stated
that GitHub redirects a renamed repo's uses: path, so a pinned-tag
consumer "keeps working unchanged / nothing breaks." That is the opposite of
GitHub's actual behavior. From GitHub's official renaming a repository docs:

GitHub will not redirect calls to an action hosted by a renamed repository.
Any workflow that uses that action will fail with the error repository not found.

git/web/API access is redirected (which is why a reusable workflow's own
inner actions/checkout of the old path still clones) — but the outer
uses: resolution is not. The old guidance would have told pinned-tag
consumers to skip the single edit they must actually make.

Changes

• docs/migration.md — short answer, the impact-by-pin table, the
  redirect-scope paragraph, and the "heron specifically" section. New truth: a
  pinned-tag consumer must change the uses: owner to Netis/olympus,
  keeping the same @tag (tags travelled with the repo) and keeping
  .agent-ops.json + agent_ops_ref until they re-pin to a post-rename tag.
• Fixes the stale "heron is not a consumer yet" section — heron is
  consumer v0.2.0: review.yml pr_number input (fix manual-dispatch re-review) #1 (5 pinned wrappers at @v0.2.0/@v0.3.1). The repoint is tracked
  in Netis/heron#164.
• docs/pantheon.md — mechanism-layer note + acceptance bullet clarified
  (git/web/API redirect ≠ Actions uses:).
• scripts/lint/check-legacy-naming.sh — header comment + the uses: fix
  message no longer claim a redirect.

Context

This lands right after the repo rename Netis/agent-ops → Netis/olympus. The
self-dogfood loop is unaffected by the rename (self-* workflows use local
uses: ./… + olympus_repo: ${{ github.repository }}); this PR's own
self-guard run is the live proof of that.

Docs/linter-message only — no behavior change. check-leakage,
check-legacy-naming, shellcheck, and the scripts/agent-bot/tests/* suite
all pass locally.

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