feat(skills): apply Add/Merge/Discard flow to miner.rs (AutoSkill A2 full integration)

[bug-ops]

Description

The AutoSkill A2 decide() three-way flow (merger.rs) is currently only wired into trace_extractor.rs. The original design (#4448) also intended miner.rs to use the same flow, but spec ambiguity meant only the trace path was implemented in the initial PR.

Expected Behavior

miner.rs dedup path calls decide() from merger.rs and handles MergeDecision::Merge (LLM-merge existing skill), Add, and Discard — replacing the current binary novel/duplicate logic.

Actual Behavior

miner.rs retains its original binary novel/duplicate check.

Found in

Code review of #4448 / PR feat/issue-4447/autoskill-a1-a2.