Minor hardcoding cleanup batch

[JSONbored]

Context

Part of the review-stack architecture audit (parent epic — hardcoding dimension). Five small,
independent, low-severity items bundled into one issue — each worth fixing for consistency but none are
currently causing live breakage:

1. isOwnReviewThreadAuthor hardcodes a gittensory-slug regex instead of GITHUB_APP_SLUG

src/github/backfill.ts:3856-3858:

export function isOwnReviewThreadAuthor(login) {
  return /^gittensory[-\w]*\[bot\]$/i.test(login ?? "") || /^(gittensory|gittensory-orb)$/i.test(login ?? "");
}

Every other "is this our own bot" check in the codebase correctly derives from env.GITHUB_APP_SLUG
(src/github/self-authored.ts:28, src/github/pr-actions.ts:131, src/github/comments.ts:59,
src/queue/processors.ts:12714,12993, src/github/backfill.ts:2517 itself) — self-hoster-configurable
via src/selfhost/setup-wizard.ts:146. Currently harmless (a renamed-slug bot's own comments fall
through to "not authorized" via other trust checks too, same false result either way) but latent —
align with the GITHUB_APP_SLUG pattern for future-proofing.

2. PROTECTED_AUTOCLOSE_AUTHORS is a closed, non-configurable set

src/settings/agent-actions.ts:59: new Set(["github-actions[bot]", "dependabot[bot]", "renovate[bot]"]).
A protective allowlist — missing entries only mean a PR from some other automation bot (e.g.
mergify[bot], snyk-bot, allcontributors[bot]) is theoretically eligible for normal duplicate/slop
auto-close heuristics, not an artificial rejection wall. Add a small env var override
(PROTECTED_AUTOCLOSE_AUTHORS_EXTRA) if this becomes a recurring complaint.

3. Orb dashboard landing-page link hardcodes gittensory.aethereal.dev

src/orb/oauth.ts:116-118 (landingPage). Judgment call: the Orb subsystem is architecturally a single
central hub by design (src/orb/broker.ts: "gittensory holds it centrally and mints on demand"), so this
is closer to GITTENSOR_HOME_URL-style "inherently one canonical service" than a per-self-hoster wall.
Flagging for consistency with PUBLIC_SITE_ORIGIN used one file-family over, and because a future hosted
product could legitimately want to run its own central hub.

4. OAuth returnTo redirect allowlist keeps a dead literal

src/auth/github-oauth.ts:246 — the allowlist unconditionally adds "https://gittensory.aethereal.dev"
as a valid redirect target even when a self-hoster has set their own PUBLIC_SITE_ORIGIN. Harmless
(additive, not exclusionary) but dead weight. Drop the literal, rely solely on siteOrigin.

5. One-off advisory finding hardcodes "jsonbored/gittensory"

packages/gittensory-engine/src/signals/local-branch.ts:807-817 — ships inside the published npm
package the generic gittensory-miner CLI depends on for any repo a miner analyzes. Advisory-only,
narrowly self-referential ("don't treat the Gittensory project itself as a mining target"). Lowest
priority — flagging only because it's a literal in otherwise-generic signal code with no config path.

Acceptance criteria

• Items 1, 2, 4 fixed as described.
• Items 3 and 5 fixed or explicitly deferred with a one-line rationale in the PR description (both
  are judgment calls, not clear-cut bugs).

[linear-code]

JSO-3391