feat(ci): track maintained fork features

[incognitojam]

Note

Adds the first incremental capability ledger and uses it to flag upstream overlap during Fork Nightly.

What changed

• adds a validated, capability-oriented fork ledger with six initial high-risk or conflict-prone entries
• distinguishes fork implementation evidence from upstream contact paths
• makes Fork CI validation blocking and Nightly overlap reporting advisory and rename-aware
• documents incremental maintenance, review, and retirement of fork capabilities

Verification

• vp run --filter @t3tools/scripts ledger:check
• vp test run scripts/fork-feature-ledger.test.ts scripts/rebase-onto-upstream.test.ts (13 tests)
• vp run --filter @t3tools/scripts typecheck
• targeted lint for the ledger scripts and tests
• git diff --check
• confirmed all 22 initial upstream contact paths exist on upstream/main

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Written by an agent (T3 Code, gpt-5.6-sol).