What this PR does / why we need it?

Implements issue #363: a reusable, agent-facing code-optimization skill under
.agents/skills/code-optimization/. This PR is skill-only; it does not change any
file under mindspeed_mm/.

How the skill works (per reviewer feedback): it does not edit code. Running it
produces a Markdown report (code_optimization_report_<date>.md) that lists every
candidate cleanup as one row of a checklist table with a 决策 column, plus for each
candidate a full diff, the ripple analysis (callers / registration / YAML / import /
tests) and a semantics-preservation rationale, and finally a section of items the
skill saw but deliberately did not propose (with reasons). Reviewers go through the
report one by one and decide which items to apply; nothing is applied automatically.

Files:

• SKILL.md — when to use, the 3-phase workflow (locate → judge → ripple + report), reject
  criteria, no-touch zones, anti-patterns.
• references/optimization_map.md — where to look first; reuses the repo's own
  fsdp2_code_boundaries.md change tiers as the no-touch baseline.
• references/finding_schema.md — per-finding field contract and the report-file layout.
• examples/code_optimization_report_example.md — a real report produced by running
  the skill on mindspeed_mm/: 8 candidates (2 structural: a byte-identical duplicate
  method and a dead private function pair; 6 zero-risk: duplicate imports and dead
  stores) and 10 deliberately skipped items, including two shadowed duplicate
  definitions with different bodies that the skill flags as suspected bugs for a
  human instead of proposing a deletion.

Related issue: https://gitcode.com/Ascend/MindSpeed-MM/issues/363

Does this PR introduce any user-facing change?

No. Documentation / agent skill only. No source, config, or training-behavior change.

How was this patch tested?

• Every diff in the example report was extracted back out of the Markdown and passed
  git apply --check against master (7a54aa84, the branch base) and against the
  current master head (60591711); all 8 applied together compile (py_compile) and
  pyflakes warnings only decrease.
• markdownlint (repo docs gate rules), codespell and trailing-whitespace checks pass on
  all files in the skill directory. pre-commit excludes *.md, so it has nothing to
  check for this PR.