feat(skills): add refactor skill and plugin manifest

[h30s]

Closes #6

What this adds

Adds the refactor skill to OxCode (plugins/refactor/skills/refactor/SKILL.md) and registers it in marketplace.json and plugin.json.

A refactor is the one change where "it still works" is the entire acceptance criterion. This skill enforces proving behaviour preservation rather
than asserting it:
- Requires a recorded green test baseline before touching any source code.
- Enforces atomic transformations with the test suite run between changes.
- Forbids folding bug fixes into a refactor ("I noticed X and did not change it").
- Requires stating test coverage gaps when touched code is untested.
- Reports the diff by structural intent (what moved, renamed, extracted, simplified, and stayed identical) rather than raw file lines.

Task tested

Refactoring a user authentication and session token handler module with mixed validation and token decoding logic.

Without skill:

The model rewrote large chunks of the file at once, silently bundled an off-by-one expiry fix that was not in the test suite, declared "all tests pass" despite the token decoding branch having zero unit tests, and outputted a massive unified diff without explaining structural transformations.

With skill:

The model ran the test suite first, recorded 18/18 passing tests as the baseline, executed three atomic extractions (verifyTokenSignature, decodeClaims, validateUserSession) running the test suite after each step, flagged "I noticed that expired tokens within a 5-second skew are rejected rather than allowed in auth.ts:42 and did not change it", explicitly noted the coverage gap for custom header decoding, and structured the final report by intent.

What was deliberately left out

• Did not allow automatic test generation inside this skill; refactoring relies on existing test baselines and inspection rather than generating untested assertions.

• Did not allow inline bug fixes during a refactor to preserve strict behavioural equivalence.

Tools value

Set to tools: full as the skill requires running tests in the workspace and applying atomic code transformations.