Rename the visual-capture-fallback.yml cross-repo contract with a compatibility alias

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Problem: .github/workflows/visual-capture-fallback.yml is an exact-name cross-repo contract, not just an internal CI file. Per the file's own comment (line ~33), any other self-hosted repo that wants to opt into visual capture has to copy this exact filename and the workflow's internal name — 'Gittensory Visual Capture Fallback' (set at line 1) — verbatim into their own .github/workflows/ directory. The workflow also defines four env vars, GITTENSORY_DIST_DIR, GITTENSORY_SERVE_PORT, GITTENSORY_ROUTES_JSON, GITTENSORY_BUILD_CMD (lines ~88-90 and 105), and produces an artifact named gittensory-visual-fallback (line ~171); it's also referenced by path from .gittensory.yml.example at line 1072. None of these four env vars are read anywhere under src/selfhost/**, so this surface sits entirely outside issue #4774's scope, which only covers the ~20 env vars actually read under src/selfhost/**. No issue currently owns this cross-repo contract, and renaming the filename, the workflow name, or the env-var set without a documented alias or grace period would silently break every downstream repo's visual-capture integration the next time it pulls from upstream documentation.

Area: Infra / CI

Proposal: Introduce the rebranded workflow name, filename, env vars, and artifact name alongside a documented compatibility alias for the current exact-name contract (visual-capture-fallback.yml, 'Gittensory Visual Capture Fallback', the four GITTENSORY_* env vars, and the gittensory-visual-fallback artifact name), so that any downstream repo already relying on the current exact names keeps working through a declared grace period, then update .gittensory.yml.example's reference at line 1072 to match.

Deliverables:

• The renamed workflow file/name/env-vars/artifact live, with a documented compatibility alias covering the current exact names for a declared grace period.
• .gittensory.yml.example's reference (currently line ~1072) updated to the new name.
• A written migration note for downstream self-hosted repos describing how and by when to move off the old exact-name contract.

Acceptance criteria:

• Downstream repos that copied the current exact-name workflow file/name continue to get working visual-capture fallback runs through the documented grace period.
• .gittensory.yml.example reflects the new workflow name/filename with no stale reference left behind.

Test scenarios:

• Confirm a repo that has copied today's exact-name workflow file still runs successfully after this change lands, unmodified, during the grace period.
• Confirm the renamed workflow runs end-to-end under its new name, env vars, and artifact name once adopted.

Resources:

• .github/workflows/visual-capture-fallback.yml
• .gittensory.yml.example (line ~1072)

Boundaries:

• The four GITTENSORY_* env vars and the gittensory-visual-fallback artifact name here are not read anywhere under src/selfhost/**, so this is a standalone cross-repo contract surface, out of scope for issue Environment variable family migration #4774's src/selfhost/** env-var audit.
• Leave every reference to the separate gittensor network's own domain/name untouched.

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Part of #4761.

[linear-code]

JSO-3798