fix(fleet): restore safe fixer and gate reproducible proof-stack builds (#512)

Restore the proof-stack work from #507 with safe filesystem writes and
fleet dispatch. Fixers reject parent-directory traversal, including
symlink traversal, and use safe no-replace behavior. Dispatch and
supervised scans honour configured fleet/repository/data roots, retain
supported legacy repository-name fallbacks, and record one outcome per
distinct store.

Skeleton emit/check reproduces the canonical, uninstantiated template
repository: canonical descriptors and anchor, project metadata
placeholders and drift detection. It is not a project initialiser.

Require locked builds, tests and Clippy for automaton, shared context
and dashboard, plus dispatch contracts and GSBot tests/dependency
security. GSBot uses Serenity's supported native TLS backend to remove
the vulnerable webpki 0.102 chain. Patch remaining fixable
unsound/yanked dependencies. cargo-deny checks the enabled graph without
vulnerability exclusions; derivative remains a visible unmaintained
warning.

Build Pages with pinned Casket source, fixed GHC/Cabal and index state.
Allowlist the exact locked Haskell setup commit. PRs validate the
artifact and link its README/docs to the checked-out merge SHA; other
events retain branch/tag links. Public deployment is constrained by the
github-pages environment's main/gh-pages policy. Replace the circular
pre-merge production deployment requirement with a required Pages build,
preserving review, scanning and existing 95% coverage policies.

Validation: containment/no-clobber, recipe-directory, skeleton
emit/check/drift and dispatch filesystem tests pass locally; GSBot
passes 18 tests. The enabled-graph audit passes, while a copy restored
to the vulnerable TLS feature fails the same auditor. Dispatch fixtures
use an inert fixer and validate orchestration only. Workflow lint and
authoritative actions-lock validation pass for the PR-link correction.
These tests do not establish 95% whole-project coverage or every fleet
bot's behavior.

Review disposition: the latest review accepted template semantics and
earlier root/path fixes. Its PR-link finding is fixed. The requested
fallback to the old Rust string "/var$REPOS_DIR" is not restored: it was
a literal malformed path, not shell-expanded configuration. Set
REPOS_BASE for older checkout locations or pass a local path. The
canonical default is developer/hyper-repos.

Normal squash auto-merge is enabled subject to repository rules. Review
5129358346 is superseded by the PR-link fix and the documented
disposition of its malformed-path fallback request; all inline threads
are resolved. Retiring that requested-change state does not assert a new
CodeRabbit approval or waive any CI rule. This PR supersedes #507 once
merged. Actions use GitHub's runtime-enforced lockfile:
https://github.com/github/gh-actions-lock#how-it-works

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Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>
Co-authored-by: Arena Agent <bot@arena.ai>

Author: 3 people