docs(agents): configure the engineering skills for this repo

[lesnik512]

Summary

/triage could not run on this repo: there was no docs/agents/ config naming the issue
tracker, the triage label vocabulary or the domain-doc homes, and four of the five triage
state labels did not exist. This is the same setup already merged for
modern-python/.github, adapted to this repo's convention.

Prompted by #32, which was transferred here from the core tracker during triage and landed
with enhancement only, because no state label existed to carry.

Changes

• Add docs/agents/issue-tracker.md (GitHub via gh; PRs as a request surface: no).
• Add docs/agents/triage-labels.md — the five canonical roles, each mapping to itself.
• Add docs/agents/domain.md, adapted rather than stock: it maps the skills'
  CONTEXT.md + docs/adr/ model onto this repo's architecture/ + planning/
  convention instead of introducing a second truth home. It also names modern-di's
  glossary as the upstream authority for shared terms (Container, Provider, Group,
  Scope), since this package is a thin adapter and should not redefine them.
• Link all three from a new ## Agent skills section in CLAUDE.md, in the bullet format
  the sibling repos use.

Out of band, on the repo itself: created needs-triage, needs-info, ready-for-agent
and ready-for-human, mirroring the colors and descriptions modern-di uses.

Notes for review

• No change file. The Quick path puts this in the Full lane (new files, multi-file),
  but planning/changes/ is empty and the last four PRs (chore: adopt ruff 0.16.0 #28, docs: drop the deprecated validate= argument, validate explicitly #29, docs(release): notes for 3.0.1 #30, Bump planning convention 2.0.0 -> 2.2.0 #31 — including a
  convention bump) shipped without one. I followed the repo's actual practice rather than
  writing the first-ever change file as a side effect of agent setup. Say the word and
  I'll add one.
• No docs/ directory existed before this. There is no MkDocs site here, so unlike
  .github no site-exclusion is needed — nothing publishes docs/.
• issue-tracker.md inherits a latent template defect: it documents filtering external
  PRs by authorAssociation on gh pr list, a field gh 2.98.0 rejects. Unreachable
  while the PRs flag is no, and the same line sits in the sibling repos — worth a
  separate sweep.

Verification

just lint-ci passes in full: eof-fixer --check, ruff format --check,
ruff check --no-fix, ty check, planning: OK, links: OK.