plugins: five modules reinvent the shared substrate — roughly a thousand duplicated lines the sixth plugin will copy again

Problem

The duplication across plugins/{codex,git,github,sql,vcs} is not incidental — the source names it, repeatedly, and always for the same reason: separate Go modules with no shared non-root package to put it in. The ledger, grep- and diff-verified:

Every plugin module already requires and replaces the root module and already imports pkg/flowstate/v1/netpolicy and pkg/flowstate/v1/secrets across that boundary, so a shared home (a plugintoolkit package, and a test-side kit under pkg/flowstate/v1/plugin for the reachable/ harness) needs no new module wiring — it is subtraction behind calls that already cross this exact seam. This is the repository's own bar for a new abstraction (remove real duplication) met with measured numbers, and it is the change that makes plugin #6 cost less to build correctly: today each new plugin re-copies the harness to earn its reachability proof and re-derives the same egress/secret/validation/bounding conventions by citation.

Conventions propagated by citation are the softer half of the same problem: sql/bounds.go and codex/bounds.go exist "for the reason plugins/vcs/validate.go gives"; three of five plugins hold README example fences byte-identical to disk via readme_test.go, two do not; sql/errors.go's own doc comment records two classification bugs of exactly the kind a shared, tested classifier pattern would have caught once.

Desired outcome

A design record deciding the extraction: which rows above land in a shared package (the test harness is the safest first slice — test-only, mechanical, largest single win), which stay per-plugin for real reasons (per-backend error taxonomies argue for a parameterized helper, not a replacement; the five test doubles are correctly not shared), and the API shapes. Explicitly not a manifest maturity/tier field — #239 settled that a self-declared danger class is a value a malicious plugin sets.

Acceptance

• The extraction lands with no behavior change: make test-plugins green, all five reachable/ suites keep their assertions (pre-registration refusal, schema refusal, spoofing proof) through the shared kit.
• The next new plugin's reachable_test.go is ~30 lines of calls, not ~100+ of copies.
• The unit inconsistency between the three installEgressPolicy copies is gone (or subsumed by #1332's grant, which should be written once here rather than five times).

Adjacency

#1330 (secrets migration removes the biggest production-code row), #1332 (egress contract owns that row's future), #713 (owns the external, language-neutral conformance harness — this is the in-tree half), #715 (ordered the outside-facing track; this is its internal complement).

Begin by reading the duplication ledger and the five plugins/*/reachable/reachable_test.go suites, then run make test-plugins to establish the current behavior. Define the shared production and test-side package boundaries and API shapes, preserving each suite’s pre-registration refusal, schema refusal, and spoofing assertions. Done means behavior is unchanged and a new reachable_test.go is about 30 lines.