feat: add rule and tags support to fme_experiment

[MatiasCasaliSplit]

What

Adds rule and tags support to the fme_experiment resource:

• create: optional rule (defaults to "default" when omitted or null) and tags ([{name}]).
• update (merge-patch): rule (null clears) and tags (replacement list; null or [] clears). Omitting either leaves it unchanged.
• list: new tags array filter (experiments having any of the given tag names); list items now keep tags and rule in the compacted output.
• Descriptions, body schemas and README updated.

Why

The v4 experiments API now exposes rule and tags on read, create and update, and a tags list filter. Without this, agents cannot scope an experiment to a targeting rule, manage its tags, or filter by tag.

Testing

• pnpm vitest run tests/registry/feature-flags.test.ts: 212 passed.
• pnpm build: passed.
• pnpm standards:check: 79 passed.
• pnpm docs:generate then pnpm docs:check: README up to date.
• Full pnpm test not run.
• Live QA over stdio against the QA backend (org anotherorg, project projectA), on an existing test experiment, restored afterwards:
  • list with tags filter: single tag and any-of ([nonexistent, existing]) return the tagged experiment; a nonexistent tag returns none. Items include tags.
  • update rule: "default": accepted and returned as "default".
  • update tags replacement, tags: [], and rule: null all behave as documented.
  • create with rule/tags was not exercised live (unit-tested only).

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