feat(validate): add explicit Tier 2 and Tier 3 gating controls

[rng1995]

Landing status

This PR merged into the staging branch naren/tier3-provider-preflight, not into main. Its base PR #19 is closed and was intentionally not merged, so the flags and reporting contract below are not available from the current GitHub release line.

Follow-up #49 replays the reviewed gating commits onto current main, adds the current Fern and repository documentation, and supersedes this PR for the release line. Review and merge #49 rather than attempting to retarget this already-merged stacked PR.

Summary

Adds explicit Tier 2 and Tier 3 gating controls and makes CLI, JSON, Markdown, and HTML consume the same finalized gating policy.

Source and dependency

• Public adaptation of internal d2ae260 from the 0.8.3 line (ed7ae8e8396ec0788135741192e6d75e4ba23724).
• Stacked on feat(tier3): add trusted coverage and provider preflight #19 for Tier 3 source/contract support.

Public-compatible behavior

• Tier 1 always blocks.
• Tier 2 remains blocking by default; --no-block-on-dedup makes it advisory.
• Tier 3 remains advisory by default; --block-on-agent-eval makes it blocking.
• Validates selected YAML/JSON or native Harbor task sources before live execution.
• Records per-tier gating, Tier 3 applicability, public profile metadata, and a source-independent policy digest.

Adaptations and exclusions

• Does not port internal/external audience switching or internal profile defaults.
• Preserves the existing public exit-code behavior unless users opt into/out of the new controls.
• Adds no managed runtime, internal provider, private endpoint, Milvus, or server behavior.

Validation

• Complete exact-lockfile suite: 3338 passed, 8 skipped, 3 deselected.
• Focused flag, source-preflight, policy-digest, and reporter-consistency tests: passed.
• Ruff, CLI golden, git diff --check, packaging, and OSS-boundary checks: passed.

Risk controls

• Reports and process exit use the same result objects after policy application.
• Missing/invalid Tier 3 sources are advisory unless explicitly configured to block.
• Network/runtime failures remain visible even when advisory.

[rng1995]

Reviewed the explicit Tier 2/Tier 3 gate selection, source preflight, policy digest, and JSON/HTML/Markdown reporting consistency. I found no blocking code issues. The focused command, source, reporter, policy, and evaluation tests passed locally (86 tests), and the GitHub check matrix is green.

[rng1995]

Review found one blocking reporter/exit consistency issue; details inline. The stacked branch is otherwise current with its declared base and has no existing unresolved threads.

[rng1995]

Ready for an independent maintainer approval. All actionable review feedback is resolved. Final head 4f67972 is conflict-free against the exact declared base naren/tier3-provider-preflight at d407bd2: GitHub reports MERGEABLE/CLEAN, the branch is 0 behind / 3 ahead, and local git merge-tree produced 1589d694503e9e78384b38389c8394b61203f74f without conflicts. Every final-SHA CI/security/package check is green. Local verification: 3,339 passed, 8 skipped, 3 deselected with 79% coverage; focused gating/source/reporting matrix 87 passed; ruff, OSS boundary, diff check, and wheel/sdist build passed. GitHub rejected my APPROVE submission because this authenticated account is the PR author and cannot approve its own pull request.