fix(sleep): avoid mutating tasks while grouping

[Yif-Yang]

Follow-up hardening for #184.

The existing dedup_tasks helper merges records in place. group_tasks_by_skill_hint now operates on copied records so grouping cannot mutate caller-owned TaskRecord instances, and it keeps each returned record skill_hint aligned with the normalized evidence used for routing.

Validation:

• pytest tests/test_sleep_engine.py -q: 95 passed, 1 skipped
• pytest tests/ -q: 614 passed, 6 skipped, 130 subtests passed
• Claude Code + Opus 5.0 independent review: safe; no blocking findings

[Copilot]

Pull request overview

Hardens the sleep miner’s task-grouping helper so it remains side-effect-free with respect to caller-owned TaskRecord instances, while ensuring each returned grouped record’s skill_hint matches the normalized evidence used for routing.

Changes:

• Update group_tasks_by_skill_hint to deduplicate/merge using copied TaskRecord instances to avoid mutating inputs.
• Normalize/align returned TaskRecord.skill_hint to the computed routing hint (including blank/ambiguous cases).
• Expand tests to assert no input mutation occurs and that returned skill_hint values are normalized/blank as expected.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
skillopt_sleep/mine.py	Copies TaskRecord inputs before dedup_tasks() and aligns returned skill_hint with normalized routing evidence.
tests/test_sleep_engine.py	Adds assertions covering non-mutation of inputs and skill_hint normalization behavior in grouped outputs.

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