Add a write-code skill for code conventions

[cristianrgreco]

Summary

Adds a write-code skill for the review finding a linter can't catch: new code that repeats something the project already has. It tells the agent to look for existing code first, to refactor shared code instead of copying it, and to use core's building blocks.

AGENTS.md ("Code"), review-pr and add-module each get a one-line pointer that only names the skill. The skill names no module, file or method, so it does not go stale as they change.

Lint-style conventions are left to Biome. #1585 and #1590 enable function length, cognitive complexity, ??, no console, no shadowing, block statements, no useless else and readonly. An earlier version of this skill also listed layout, typing and naming conventions. They were removed because Biome has a rule for them, or they were too minor to be worth a rule.

What remains, with the evidence for it (survey of the 191 non-test source files, taken on main before #1514 merged):

Rule	Evidence
Look for existing code first, and refactor it rather than copy it	"apply the startup timeout, then waitForContainer" appears 5 times across core, Kafka and Redpanda; the two module hooks repeat the same 10-line block. #364
Where shared code goes: a helper in the file, a function in the package, core for anything two modules need	modules import only testcontainers (enforced by Biome), so cross-module reuse has to live in core
Helper defined below its first caller	77 after, 29 before. 14 of the 29 were in couchbase-container.ts
with* methods rather than createOpts or hostConfig	2 direct assignments in modules. #1155
Wait with a WaitStrategy through waitForContainer	Kafka and Redpanda do. Couchbase calls waitUntilReady directly and is the only module that polls with IntervalRetry. #723
Post-start waits share what is left of the startup timeout	Couchbase since #1514. #1584 proposes a core StartupDeadline for it

When #1584 merges, the timeout bullet should name the argument it adds to containerStarted.

Verification

• npx biome check .: no fixes applied.
• npm run knip: no findings.

The change is Markdown plus the .claude/skills/write-code symlink, so there are no tests to run.

Not breaking

Agent guidance only. No package code or manifests change.