[Feat] Canonical @roomote mention alias on GitHub

[mrubens]

Product decision: user-facing copy always advertises @roomote, and every deployment listens for both @roomote and its own configured GitHub App slug. Roo Code owns the roomote GitHub App and user, so the alias cannot be claimed by a third party.

• isMention matches the canonical alias or the effective slug, keeping the word-boundary semantics from [Fix] Require word boundaries when matching GitHub app mentions #238.
• PR provenance footers now advertise @roomote (new ROOMOTE_CANONICAL_GITHUB_MENTION constant) instead of the deployment slug.
• R_GITHUB_DISABLE_CANONICAL_MENTION=true opts a deployment out: it stops answering bare @roomote and its footers advertise its own slug again. For fleets where several deployments share the same repositories (e.g. nightly + preview on one org) so a bare @roomote doesn't make all of them respond. Documented on the GitHub provider docs page.
• With the footer canonical by construction, the write-time attribution rewriter (normalizePrBodyAttributionAppMention) and its slug-resolution helper (resolveConfiguredGitHubAppSlugIfConfigured) are obsolete and removed, along with their tests and the sdk call site.
• about-me prompt and GitHub provider docs updated; stale "hosted-product" wording on ROOMOTE_GITHUB_HOSTED_APP_SLUGS freshened.

Setup flow is untouched: it already suggests roomote-<host> at app creation and shows the deployment's real handle in invoke-method copy, which stays correct alongside the alias.

Validation: targeted suites green across env (65), github (31), api, sdk, and the full cloud-agents workflow suite; check-types clean on all affected packages; prettier and knip clean.

[roomote-roomote]

No new code issues found. See task

Re-reviewed the only new commit since the last review (fix: cast mocked env through unknown in mention-alias tests). The delta is test-only: it widens the Env cast in two mention-alias test files from Env as Record<string, string | undefined> to Env as unknown as Record<string, string | undefined> so the mocked env can be mutated without tripping the compiler's insufficient-overlap error. This is the standard double-cast pattern, matches the surrounding test setup (per-test beforeEach/afterEach cleanup already restores R_GITHUB_DISABLE_CANONICAL_MENTION), and touches no production code or behavior. Both files are within the PR's changed-files set.

_{Reviewed 41313c5}