fix(inbox): hide audited merged runs

[tiammomo]

Closes #86

What changed

• Pass RepoSteward's latest native merge outcomes into the prompt-free maintainer inbox.
• Suppress a tracked submitted PR only when it is absent from the fresh open-PR portfolio and its latest terminal outcome is exactly merged or already_merged.
• Keep open PRs, incomplete facts, failed or unknown outcomes, and closed-unmerged work visible as attention items.
• Document the terminal suppression boundary and add pipeline-level and behavior regression tests.

Why

Portfolio intentionally lists only open pull requests. Previously, every submitted run whose PR was no longer open became a P80 refresh_required item even when RepoSteward had already recorded an authoritative merge result. That made historical terminal work outrank current review and merge tasks.

Validation

• Focused inbox tests: 7 passed
• Full suite: 373 tests passed
• Ruff lint and format checks, CLI smoke test, package build, and hardened verifier passed
• Live read-only smoke test on this repository: 22 attention items reduced to 6; all 16 audited merged PRs disappeared while four active Draft PRs, one failed run, and one proposal review remained
• No additional GitHub requests, Harness invocation, workspace mutation, or public write in the inbox path

Review focus

• Fresh open-PR facts take precedence over local terminal history
• Only the native merged and exact-head already_merged outcomes suppress an item
• Missing, blocked, failed, unknown, and closed-unmerged states continue to fail closed

Automation disclosure

An external coding workspace assisted with the implementation and tests. tiammomo reviewed the final diff, understands it, and takes responsibility for this contribution.