docs(selfhost): clarify checkRunMode duplicates the required gate check

[JSONbored]

Summary

• Closes out the docs follow-up explicitly offered in issue chore(selfhost): investigate duplicate check-run surfaces in one-shot review mode #2691's investigation comment. That
  investigation confirmed checkRunMode already defaults to off, that the advisory Gittensory Context check and the required Gittensory Orb Review Agent gate check are fully independent
  with zero cross-gating, and that at the default checkRunDetailLevel: minimal the Context check
  never posts findings, while at standard/deep it only re-renders findings already shown in the
  gate check and PR comment. The only remaining action item from that comment was documentation:
  nothing in .gittensory.yml.example explained that enabling checkRunMode adds a second,
  mostly-redundant check surface once the gate is already on.
• Adds a doc comment directly above checkRunMode: off in .gittensory.yml.example explaining
  that it's an independent, legacy advisory surface, distinct from and duplicative of the required
  gate check and PR comment, and that one-shot self-host installs should leave it at the default
  off unless they specifically want a second, low-detail check-run surface.
• Checked config/examples/README.md: it is deliberately scoped to the private-config
  layering/merge mechanism (precedence chain, deep-merge semantics) and explicitly defers to
  .gittensory.yml.example as "the exhaustive, field-by-field reference (not duplicated here, so
  the two never drift out of sync)." Adding a per-setting cross-reference there would duplicate
  content the file intentionally avoids duplicating, so it is left untouched.
• No code or behavior change; docs only.

Scope

• The PR title follows type(scope): short summary Conventional Commit format, for example fix(api): restore profile access checks.
• This PR is focused and does not mix unrelated backend, UI, MCP, docs, dependency, and deploy changes.
• This follows CONTRIBUTING.md and does not reintroduce GitHub Pages, VitePress, site/, or CNAME.
• I linked an issue, or this is small enough that the summary explains why an issue is not needed.

Validation

• git diff --check
• npm run actionlint (N/A — no workflow files changed; docs-only edit to .gittensory.yml.example)
• npm run typecheck
• npm run test:coverage locally (N/A — no src/** lines changed; ran the targeted example-config parse test instead: npx vitest run test/unit/focus-manifest.test.ts -t "gittensory.yml.example", passing, confirming the added comment doesn't break YAML/comment parsing)
• npm run test:workers (N/A — no worker code changed)
• npm run build:mcp (N/A — no MCP code changed)
• npm run test:mcp-pack (N/A — no MCP code changed)
• npm run ui:openapi:check (N/A — no API/schema changes)
• npm run ui:lint (N/A — no UI files changed)
• npm run ui:typecheck (N/A — no UI files changed)
• npm run ui:build (N/A — no UI files changed)
• npm audit --audit-level=moderate (N/A — no dependency changes; ran during this session's npm ci with 0 vulnerabilities found)
• New or changed behavior has unit/integration tests for new branches, fallback paths, and sanitizer boundaries (N/A — comment-only change, no new logic branches)

If any required check was skipped, explain why:

• This is a docs-only change (a single YAML comment block added to .gittensory.yml.example), so
  the code-oriented checks above (lint/build/coverage/audit/workers/mcp) don't apply. Per the task
  scope, the two checks that matter for this change — the example-config parse test and
  npm run typecheck — were both run locally and passed.

Safety

• No secrets, wallet details, hotkeys, coldkeys, user PATs, private keys, raw trust scores, private rankings, or private maintainer evidence are exposed.
• Public GitHub text stays sanitized, low-noise, and does not imply compensation guarantees or optimization tactics.
• Auth, cookie, CORS, GitHub App, Cloudflare, or session changes include negative-path tests. (N/A — no such change)
• API/OpenAPI/MCP behavior is updated and tested where needed. (N/A — no API/OpenAPI/MCP change)
• UI changes use live API data or real empty/error/loading states, not production mock/demo fallbacks. (N/A — no UI change)
• Visible UI changes include a UI Evidence section below with JPG/JPEG or PNG screenshots... (N/A — no visible UI change; this is a config-example comment)
• Public docs/changelogs are updated where needed; changelogs are only edited for release-prep PRs. (.gittensory.yml.example updated; CHANGELOG.md untouched)

UI Evidence

N/A — no visible UI, frontend, or rendered-docs change; this PR only adds a comment block to .gittensory.yml.example.

Notes

• Closes chore(selfhost): investigate duplicate check-run surfaces in one-shot review mode #2691