Extract shared buildAppOctokit() helper (dedupe 3 copies of App-auth Octokit construction)

Follow-up from PR #113 review (nit #2).

The App-authenticated Octokit construction now exists in three places:

• packages/outpost/shared/src/platforms/github.ts (GitHubAdapter.getOctokit, added in #113)
• packages/outpost/shared/src/integrations/github-client.ts
• apps/github-app/src/lib/github-client.ts

All three do essentially:

new Octokit({ authStrategy: createAppAuth, auth: { appId, privateKey, installationId } })

Proposed

Extract a shared buildAppOctokit(creds) helper (e.g. in packages/outpost/shared) and have all three call sites use it. Normalize the installationId type while doing so — it's number in the platform adapter vs string in the other two builders (nit #3; createAppAuth tolerates both, but a single helper should pick one).

Not urgent

Each copy uses a slightly different minimal Octokit interface, so this is a cleanup/DRY task, not a bug. Defer-friendly.

Compare the three construction sites in packages/outpost/shared/src/platforms/github.ts, packages/outpost/shared/src/integrations/github-client.ts, and apps/github-app/src/lib/github-client.ts, then inspect how shared code is organized. Done means the three sites use one shared buildAppOctokit helper and the installationId type is consistent across them.