refactor: consume Plugin-owned Browser and Agent implementations

[kinzhi]

Summary

• Keep CLI's single executable and command/MCP composition surface while consuming Plugin-owned Browser/Agent implementations.
• Pin the publicly resolvable Plugin module v0.1.58-0.20260905174434-1fea781f2b48 with its verified canonical checksum.
• Record the repository and Go-import compatibility changes in the changelog. The SDK Worker remains the sole Runtime lifecycle/ACK/spool implementation.

Dependency publication gate

Plugin source PR: OpenLinker-ai/openlinker-plugin#11

The original Plugin commit 1fea781f2b4876bce010d70ed88aa861405fbc23 is preserved. An independent neutral Go module with no pre-trusted Plugin sums fetched it through https://proxy.golang.org, with sumdb enabled and all private bypasses disabled. The official source hash and both checksums match the CLI manifests; signed sumdb lookup record 62227706 was retained.

Verification

• Fixed clean CLI source 4cbe94db5d30608855cb760c09d5be46723072a9: public-module GOWORK=off race tests (26 top-level entries, 4 packages, zero skips/failures/races), go mod verify and go vet pass.
• Nested example: verify/vet and one race test pass; one context test explicitly skips because the example has no embedded CLI binary. No real Provider was invoked.
• Local migration candidate six-target builds and functional Caller/config/MCP/Worker differential checks passed in the preceding acceptance. Release binaries must be rebuilt for the final main commit and injected release version, not labelled with those candidate digests.
• Root default-workspace six-module build and CLI tests pass after the temporary bootstrap replacement is retired; public module evidence remains separate from workspace use.

Merge with a merge commit; no squash/rebase. This PR does not publish native images, change Runtime volumes or deploy twv1. Compatible CLI release artifacts and a regenerated Plugin CLI lock precede those separate gates.