Adopt build conventions

[ausimian]

Brings Castle onto the organisation's Elixir build conventions, mirroring the
structure Forecastle adopted in ausimian/forecastle#5.

mix.exs

• @version and @source_url as the single sources of truth, with
  VERSION_OVERRIDE so CI can inject a pre-release version.
• precommit alias plus preferred_envs, so validation runs through a single
  gate rather than piecemeal checks.
• credo and publisho added, a default .credo.exs, and ex_doc bumped to 0.39.
• Minimum Elixir raised to 1.18, one minor below the 1.19.5 used for tooling.
• Explicit packaged file list, description moved up to project/0, and
  source_ref pinned to @version so generated docs link at the released tag.

CI

The organisation's build components are GitLab CI components and do not apply
here, so the pipeline mirrors Forecastle's: a precommit job on the preferred
toolchain, and a test matrix over the supported Elixir and OTP range
(1.18/27, 1.19/27-28, 1.20/27-29).

The matrix is Linux-only for now — Castle is pure Elixir talking to
:release_handler, with no shell scripts of its own since 0.3.0 and no suite
that boots a release, so there is nothing platform-sensitive to run. Until
there are tests, the matrix earns its keep on the deprecation surface, which
matters for code reaching into OTP release internals.

Publishing triggers on bare semver tags, as produced by mix publisho, and
refuses to publish if the tag and @version disagree.

Note

The publish workflow needs a HEX_API_KEY repository secret. This has now
been set on the repository.

Source changes

credo --strict findings only, no behaviour change: parentheses dropped from
the zero-arity definitions, a redundant blank line removed, and the config
provider called directly rather than through apply/3 — the function name and
arity are known, only the module varies.

Tests

mix test failed before this branch: the generated test asserted on
Castle.hello/0, which has never existed, so there was no working gate to
adopt. It is now a doctest stub.

This does not give Castle test coverage, and #8 stays open. Every function
here drives :release_handler against an installed release, and generate/1
resolves paths from :code.root_dir(), so exercising any of it needs a release
fixture booted in a workspace, as Forecastle's :e2e suite does. Forecastle's
upgrade_test.exs covers this code end to end in the meantime.

Docs

AGENTS.md describes what Castle does, how it pairs with Forecastle, and the
conventions for working on it, with CLAUDE.md importing it. It records the
known limitations and the issues tracking them (#8, #9, #10, #11) so they are
not rediscovered. RELEASE.md carries the raised Elixir requirement, the only
consumer-visible change here. CHANGELOG.md gains the publisho placeholder;
existing entries are left as published.

Refs: #8

[ausimian]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Keep them coming!

Reviewed commit: b70b0e85d7

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