refactor: remove Phase 8 compatibility paths

[eshulman2]

Phase 8 implementation plan: compatibility removal

Status: Complete

Goal: Delete superseded execution paths so Forge has one runtime model rather than
permanent legacy and contract-backed implementations.

Removal rule

A compatibility path may be deleted only when its replacement is authoritative for all
golden paths, restart/replay characterization passes, persisted state has an explicit
migration policy, and rollback does not require the deleted implementation. Phase 8 is
not permission to remove behavior that an earlier partial phase has not replaced.

Completed cutovers

The Jira and source-control worker handler facades are deleted. Since Phase 2, both
sources register the same generic adapter-driven handler; the source-specific methods had
no runtime or test callers and represented a second, misleading dispatch API.

Phase 8 also removes the legacy Redis stream and github source alias, implicit
checkpoint pinning, scalar planning fallbacks, the implementation-input facade, and
repository-key fallback migration. Built-in runtime selection is definition-compiled;
the Python graph adapters remain only as local test harnesses. Architecture tests make
these removals zero-tolerance.

Unpinned checkpoints must now be processed by migrate_unpinned_checkpoint. Operators
first run it with apply=False, retain the original checkpoint as the rollback backup,
and persist the returned migrated_state only when compatible is true. Applied state
records the target definition and a seven-day rollback deadline by default. Rollback
means restoring that backup before the deadline; normal resume never performs migration
or rollback implicitly.

Final observation cutover

CI, merge, review-thread, and proposal-review observations are now applied by the
provider-neutral post-pr-v1 transition policy. The pinned workflow definition selects
that policy through an allowlisted identifier; compilation rejects unknown policies and
policies whose target nodes are absent. The worker adapts ingress, delegates once, then
persists the result—it no longer owns event-specific transition rules.

The inventory at docs/architecture/phase-8-removal-inventory.json is the reviewable
exit checklist. Phase 8 is complete only when remaining is empty and the associated
architecture tests and golden-path characterization suite pass.