fix(fork-features): mark remove-hashline as dropped in manifest #20930

Task Description

The remove-hashline entry in .fork-features/manifest.json has "status": "active" but the feature was fully removed in issue #360. This is misleading — "active" implies the code exists, but all 8 hashline files were deleted (1,921 lines removed). Status must be changed to "dropped" with a reason recorded.

Background

• Hashline was a fork-only experiment (Feb 19 – Mar 15 2026): content-addressed line editing via xxHash32
• Removed due to zero agent adoption and test failures
• Upstream (anomalyco/opencode) never had hashline — no re-introduction risk
• The absorptionSignals in the entry are inert (upstream will never match them)
• Historical record of the deletion should be preserved as "dropped" (not deleted from manifest)

Quality Gates (Non-Negotiable)

• Local Verification: bun run typecheck and bun test pass locally before completion

Acceptance Criteria

• remove-hashline entry has "status": "dropped"
• "droppedReason" field added explaining when and why it was removed
• No other manifest entries modified
• bun test .fork-features/verify.ts passes (verifier skips dropped entries)