docs: commit the agent configuration (AGENTS.md, .agents/skills, .claude symlinks)

[junwen94]

Commits an agent configuration written for this repository, and stops
gitignoring it.

goldilocks-ml and goldilocks-core both commit .agents/ and AGENTS.md;
this repository listed them in .gitignore, so anyone other than the original
developer — and any agent — started here with no project instructions and no
skills.

The local copy could not be committed as-is

It was a half-adapted copy of goldilocks-core's configuration:

File	Problem
AGENTS.md	opened with # goldilocks-core and described core's Load → Analyse → Advise → Select → Generate pipeline, which does not exist here
github-cli/SKILL.md	hard-coded --repo stfc/goldilocks-core in ~15 commands, so an agent files issues against the wrong repository
catchup/SKILL.md	same, in 4 commands
write-a-test/SKILL.md	ran coverage as --cov=goldilocks_core
write-docs/SKILL.md	inspected src/goldilocks_core, referenced docs/architecture.md (absent), documented a goldilocks-kmesh CLI (the console script is goldilocks-data), and invoked a mermaid validator at /home/sigil/.pi/...
dft-basics/SKILL.md	described itself as being for goldilocks-core

What is here

AGENTS.md — written for goldilocks-data: the generate/analyse/publish
sequence, ecosystem position, the package/campaign boundary ("a private path
must never reach the package API"), the real layout, and the commands from
README.md including ruff check src tests campaigns/qe/kpoints/scripts and
the --extra aiida --extra kmesh campaign invocation.

It also records the k-index convention, which cost real effort to establish:
k_index is 0-based with rung 0 the Γ-only (1, 1, 1) mesh; the ladder comes
from the k-distances where ceil(|b_i| / k_distance) changes; and the
enumeration cap on n is applied per axis, so the ladder is a complete set of
meshes only for k_distance >= max(|b_i|) / n_max and silently skips reachable
meshes below that. Any recomputed or published k_index column must state the
cap it used.

.agents/skills/ — 12 skills, every goldilocks-core reference corrected.
write-docs is rewritten against the real docs tree and now warns that a new
page must be added to the mkdocs.yml nav or --strict fails. use-uv gains
an Extras section: AiiDA and pymatgen are optional extras, so an AiiDA import
error usually means a missing --extra, not a broken environment. triage is
ported from goldilocks-ml with its ML-specific classification rule replaced by
a campaign/package one.

.claude/skills/<name> — relative symlinks to ../../.agents/skills/<name>,
stored as git mode 120000, the mechanism goldilocks-ml uses. Codex reads
AGENTS.md and .agents/; Claude Code discovers .claude/skills/; both read
the same files, so they cannot drift.

No CLAUDE.md — neither sibling repository has one, and .gitignore still
excludes it. The existing local one pulled all twelve SKILL.md files into
every session's context through @ imports; the symlinks do the same job
without that cost.

Verification

• No goldilocks-core / goldilocks_core / goldilocks-kmesh reference and no
  foreign absolute path survives, except two deliberate ecosystem
  cross-references in AGENTS.md
• All 12 skills have a matching symlink, and all 12 resolve to a SKILL.md
• No code, test, or packaging file is touched

Closes #23

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Written by an agent on behalf of @junwen94.