Retire the three unreachable postAiResponse helpers

[NathanTarbert]

Three fully-formed "post the AI answer to the platform" helpers exist and none of them is reachable:

• apps/github-app/src/lib/github-poster.ts:36 — postAiResponse → postIssueComment / postDiscussionComment, real Octokit calls
• apps/slack-bot/src/lib/slack-poster.ts:16 — postAiResponse → client.chat.postMessage
• apps/teams-bot/src/lib/teams-poster.ts:26 — postAiResponse → context.sendActivity (the file documents its own unreachability at lines 23-24)

Nothing imports any of them except github-poster.test.ts, so there's no live leak. The real path is the worker's PlatformAdapter, which is gated on shadow mode in packages/outpost/queue/src/handlers/ai-response.ts.

The problem is what happens next time someone needs a direct post. Each of these is one import away from being wired up, and none consults shadow mode, so the caller would silently bypass the gate — and docs/deployment.md:212 currently asks the author to remember by hand. That's the same shape as the bug #233 is fixing: enforcement by convention rather than by the predicate.

Two ways to go, and I lean toward the first:

1. Delete them. The PlatformAdapter path in the worker is the real one, and dead code that looks live is worse than no code.
2. Guard them, so a future caller inherits the check:

export async function postAiResponse(target: PostTarget, options: PostResponseOptions): Promise<void> {
    if (isShadowMode()) {
        console.log(`[GitHub App] Shadow mode — response for ${owner}/${repo} withheld`);
        return;
    }
    ...
}

Worth confirming nobody has a branch in flight that intends to use them before deleting. Depends on #233 if we take option 2.

[linear-code]

CPK-8285