[P2][CODE-01] Repositories, worktrees, tests and patch review

Programme: #1
Epic: #24

## Objective
Provide a safe shared coding workspace for local/external agents: scoped repositories, isolated worktrees/builds, deterministic tests and human-reviewable patches.

## Scope
- Repository/project registration with allowed roots and protected paths.
- Per-agent/task worktrees or equivalent isolated mutable copies.
- Editor/diff/patch artifact model and merge/rebase conflict workflow.
- Isolated build/test/lint/typecheck runners with resource/network/dependency policy.
- Dependency change detection and review.
- Independent test execution after provider reports completion.
- Patch review UI with provenance by agent/task/provider.
- Merge/apply operation through scoped write capability.

## Security rules
- Protected runtime/policy/security paths can be configured read-only or require stronger review.
- Builds never inherit ambient host secrets/network.
- Agent output is untrusted until verified.

## Acceptance criteria
- [ ] Patch applies cleanly to the intended base or produces explicit conflicts.
- [ ] Independent tests execute in the permitted isolated environment.
- [ ] Parallel agents cannot silently overwrite each other's worktrees.
- [ ] Protected paths remain unchanged without an explicit stronger grant.
- [ ] New dependencies/network requests are visible and policy-controlled.
- [ ] Review shows diff, tests, provenance and verification state before merge.

## Dependencies
- CORE-02
- SEC-04

**First phase:** P2
**Maturity target:** P3
**Owner:** harness-pc-providers

No files, tests, or implementation entry points are identified. Start by reading the dependencies CORE-02 and SEC-04, then define the first-phase boundary around scoped workspaces, isolated test execution, protected paths, dependency visibility, and patch review; done means the listed acceptance criteria are demonstrably satisfied.

Written by the indexing model from the issue text.