fix(agent): keep public commands repo-scoped

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Motivation

• Public GitHub comment commands could fall back to the contributor's full private decision pack and surface a private cross-repo recommendation in a public comment, violating the public-safe requirement.
• The orchestrator needs to restrict fallback behavior for github_comment surface runs so public responses never expose unrelated private repo rankings.

Description

• Preserve the request surface when creating blocker runs by setting surface from the incoming input instead of always using "api" in explainBlockersWithAgent (src/services/agent-orchestrator.ts).
• Prevent public github_comment runs from falling back to the full pack.repoDecisions when the requested repoFullName has no matching decision by computing allowCrossRepoFallback and using scopedDecisionActions for decision selection (src/services/agent-orchestrator.ts).
• Add an options.allowFallback flag to buildBlockerActions so blocker responses for public surfaces return no actions when there is no repo-specific decision, avoiding selecting unrelated private blockers (src/services/agent-orchestrator.ts).
• Add a regression unit test that seeds a private repo decision, invokes planNextWork/explainBlockersWithAgent with surface: "github_comment" for a missing public repo, and asserts the public comment and returned actions do not reveal the private repo or recommendation (test/unit/agent-orchestrator.test.ts).

Testing

• Ran npm exec vitest -- run test/unit/agent-orchestrator.test.ts --reporter=verbose and the unit test suite passed (all tests in the file succeeded).
• Ran npm run typecheck (tsc --noEmit) and the TypeScript typecheck passed without errors.
• Ran git diff --check to ensure no whitespace/diff issues surfaced and no automated checks failed.

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Codex Task