Implement Sub-Task 5: fetch implementation PR metadata and reviews

[afrittoli]

Summary

• Adds scripts/fetch_impl_prs.py (Sub-Task 5): reads raw/teps.jsonl, extracts every (repo, pr_number) implementation PR link across all TEPs (not a curated Pass 1 sample — matches the "run all" precedent Sub-Task 4 established), and fetches PR metadata + reviews + review comments from the GitHub REST API generically across whatever tektoncd/* repos are actually linked (13 repos observed, not just the 7 named in the plan's context note).
• 404s (deleted/transferred PRs) are recorded as {"repo", "pr_number", "status": 404} rather than dropped, per the plan.
• Every record (fetched or 404) carries discovered_via: "tep_file_link", per the plan's spec — this is the only discovery mechanism implemented so far; Sub-Task 6 (cross_repo_search.py, not yet built) will later tag its own org-wide-search finds "search", making the under-linking rate measurable.
• Adds reports/impl_prs_report.html with an "Implementation PRs by TEP" table — one row per TEP listing its linked implementation PR(s), each with a review-decision badge (or 404/not-fetched badge) — plus coverage, per-repo breakdown, and a flat PR list. Matches the HTML-report pattern fetch_tep_prs.py established for Sub-Task 4.
• Adds scripts/build_report_index.py: a tabbed index page (reports/index.html) that embeds every reports/*.html in an iframe behind a tab, labelled from each report's <title>. Reports are discovered by scanning the directory, so a new report needs no index update. New make report-index target.
• Ran make fetch-impl-prs for the full set: 254 unique implementation PRs fetched, 0 not found, 5740 review comments — committed to raw/impl_prs.jsonl / raw/impl_pr_reviews.jsonl.
• Also fixes the one remaining TEP-0192 reference in pyproject.toml's description, missed in the earlier renumbering pass (Update TEP-0192 references to TEP-0173 #13).

Test plan

• uv run ruff check scripts/ tests/
• uv run ruff format --check scripts/ tests/
• uv run mypy scripts/
• uv run pytest --cov=scripts --cov-report=term-missing (96 passed)
• npx markdownlint-cli "**/*.md" (matches CI's markdownlint-cli2 config)
• Verified reports/index.html and the TEP-mapping table (spot-checked TEP-0084, which has 7 linked chains PRs) — structural/HTTP check only, did not visually drive the tab-click interaction in a real browser
• Verified all 254 existing raw/impl_prs.jsonl records were backfilled with discovered_via (no records missing it), without re-hitting the GitHub API