feat(init): scaffold pinned codebase fixtures

[slowdini]

Closes #265

Why

Full-codebase evals need a useful starting point before an author has chosen or
published a project fixture. A pinned public example makes the first scaffold
runnable, while explicit source options let established suites record the project
they intend to measure.

What changed

• Make Weeknight the default
  eval-magic init codebase, pinned to
  b6d269c1cdedf7cadb53bacc41acaf5f2cdbe03f.
• Add --codebase-url with required --codebase-ref, --codebase-path, and
  --codebase-cwd source selection.
• Resolve relative local paths from the invocation directory and record them
  relative to the generated evals/ directory. Preserve canonical absolute paths.
• Extend CLI help, the README, and the shipped eval-magic docs codebase guide with
  source selection, fixture sizing, pinning, and provenance guidance.
• Document eval-magic itself as the complex fixture for tasks that need its Rust,
  schema, integration-test, and repository-instruction surface.
• Add CLI integration tests for the default, explicit remote, relative path,
  absolute path, current-directory, conflict, validation, and help contracts.

Behavior and boundaries

With no source option, init writes:

{
  "codebase": {
    "url": "https://github.com/slowdini/eval-magic-fixture",
    "ref": "b6d269c1cdedf7cadb53bacc41acaf5f2cdbe03f"
  }
}

Remote URL/ref values are recorded without contacting the remote; run remains the
point that resolves and validates them. URL and ref must be supplied together, and
the remote, path, and current-directory modes conflict with one another. Local
sources must exist and be directories before init prompts for seed fields.

This extends the established singular codebase guide instead of adding a parallel
codebases topic. Existing revision-mode provisioning, snapshot/promotion behavior,
and source plus resolved-SHA propagation remain shared with the new-skill path and
are covered by the codebase run suites.

Examples

# Use the pinned Weeknight default.
eval-magic init

# Record another Git source without resolving it yet.
eval-magic init \
  --codebase-url https://github.com/slowdini/eval-magic \
  --codebase-ref e30a5091c844e07f3aa664413aa7735a11b0a52a

# Use the project in which init is invoked.
eval-magic init --codebase-cwd

Verification

• cargo fmt --check
• cargo build
• cargo test — 926 unit, 209 CLI, and 199 run tests passed
• cargo clippy --all-targets -- -D warnings
• git diff --check
• Resolved the public fixture anonymously at the pinned SHA.
• Ran default eval-magic init and model-free eval-magic run preparation against
  the published URL; both conditions were clean, remote-free repositories with
  matching baseline and HEAD refs.
• Fixture CI
  passed at the pinned SHA.