feat(ci): package-registry provenance (#94)

## Cargo package-registry provenance

crates.io has no native OIDC/trusted-publishing provenance, so this
attests the **built `.crate`** via GitHub build-provenance attestation.

### Changes to \`.github/workflows/publish.yml\` (additive only)
- Added \`id-token: write\` and \`attestations: write\` to the
\`publish\` job permissions (kept existing \`contents: read\`).
- Added a real \`cargo package\` step (the workflow previously only ran
\`cargo package --list\`, which does not emit a \`.crate\` artifact) so
\`target/package/echidnabot-<ver>.crate\` exists.
- Added
\`actions/attest-build-provenance@e8998f949152b193b063cb0ec769d69d929409be
# v2\` to attest \`target/package/*.crate\`, placed before publish.

### Notes
- Single-crate package (\`echidnabot\`), default manifest path, no
workspace / custom working-directory — \`cargo package\` cleanly
produces \`target/package/echidnabot-0.1.0.crate\`.
- Publish flow (dry-run vs real on tag) is unchanged.
- Pre-existing caveat (out of scope, not touched): \`Cargo.toml\`
carries a path dependency on \`../../shared-context\` that only resolves
inside the gitbot-fleet monorepo; the publish workflow's standalone
\`cargo build\`/\`cargo package\` already depends on that being
resolvable. No regression introduced here.

Author: hyperpolymath

.github/workflows/publish.yml

@@ -27,6 +27,8 @@ jobs:
     timeout-minutes: 15
     permissions:
       contents: read
+      id-token: write
+      attestations: write
     steps:
       - name: Checkout
         uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v4
@@ -46,6 +48,12 @@ jobs:
         run: cargo test --lib
       - name: Verify crate can be packaged
         run: cargo package --list
+      - name: Package crate
+        run: cargo package
+      - name: Attest crate provenance
+        uses: actions/attest-build-provenance@e8998f949152b193b063cb0ec769d69d929409be # v2
+        with:
+          subject-path: 'target/package/*.crate'
       - name: Publish to crates.io (dry run)
         if: github.event.inputs.dry_run == 'true'
         run: cargo publish --dry-run