fix: stop system tasks from minting per-user agent rows; add agent prune

[chubes4]

Closes #2864.

Problem

datamachine_resolve_system_agent_context() conflated attribution (which human triggered the work) with agent identity (a persistent row in the agents table). Any authenticated user who triggered a system-owned queued task — uploading an image, generating alt text, meta descriptions, image optimization, internal linking — got a full agent identity minted from their login. ChatOrchestrator is the only legitimate auto-provision surface.

Fix: attribution vs. identity

• datamachine_resolve_system_agent_context() now always resolves to the install's default agent owner (DirectoryManager::get_default_agent_user_id() → its agent id). It only calls datamachine_resolve_or_create_agent_id() for that default owner, never for the triggering human.
• The original triggering user is returned separately as triggering_user_id.
• Media/SEO/linking callers pass triggering_user_id into the TaskScheduler context so it is preserved in engine_data['task_context'] for audit, without minting a row.
• ChatOrchestrator keeps datamachine_resolve_or_create_agent_id() — opening chat is still operating an agent.

Cleanup: wp datamachine agent prune

• New datamachine/prune-agents ability and matching wp datamachine agent prune subcommand.
• Default is dry-run listing; --yes applies deletion.
• A candidate is an agent row with zero references:
  • no chat sessions (network-wide table + legacy per-site tables)
  • no jobs (agent_id)
  • no access grants (user or principal)
  • no on-disk agent directory
  • no datamachine_bundle config block
  • not the install's default agent
• Uses the existing deleteAgent path so grants are cleaned up; directories are never auto-deleted.

Tests

• Updated tests/system-agent-context-resolution-smoke.php for the new resolver behavior.
• Added tests/agent-prune-smoke.php covering zero-reference detection, default-agent protection, and session/job/grant/directory/bundle spares.

Verification

• Local phpcs clean.
• homeboy review data-machine --changed-since origin/main --summary: lint passed; test stage failed before running tests due to a missing wp-codebox CLI binary in this environment (infrastructure, not code).
• Manual smoke-test runs pass.

Fixes #2864

[homeboy-ci]

Homeboy Results — data-machine

Lint

✅ review lint — passed

ℹ️ Full options: homeboy self docs commands/lint
Deep dive: homeboy review lint data-machine --changed-since ad09ae9

Artifacts and drill-down

• CI results artifact: homeboy-ci-results-data-machine-review-lint-quality-Linux-node24 contains immediate command JSON for this action invocation.
• Observation artifact: homeboy-observations-data-machine-review-lint-quality-Linux-node24 contains exported Homeboy run history for deeper queries.
• Drill-down: download the observation artifact, then run homeboy runs import <dir>, homeboy runs list, and homeboy runs findings <run-id>.
• Artifacts are attached to the workflow run: https://github.com/Extra-Chill/data-machine/actions/runs/28970015680

Test

❌ review test — failed

ℹ️ No tests ran — the runner failed before producing results. See raw_output.stderr_tail / raw_output.stdout_tail for the underlying error (bootstrap failure, missing deps, DB connection, etc.).
ℹ️ To run specific tests: homeboy test data-machine -- --filter=TestName
ℹ️ Auto-fix lint issues: homeboy refactor data-machine --from lint --write
ℹ️ Collect coverage: homeboy test data-machine --coverage
ℹ️ Analyze failures: homeboy test data-machine --analyze
ℹ️ Pass args to test runner: homeboy test -- [args]
ℹ️ Full options: homeboy self docs commands/test
Deep dive: homeboy review test data-machine --changed-since ad09ae9

Artifacts and drill-down

• CI results artifact: homeboy-ci-results-data-machine-review-test-quality-Linux-node24 contains immediate command JSON for this action invocation.
• Observation artifact: homeboy-observations-data-machine-review-test-quality-Linux-node24 contains exported Homeboy run history for deeper queries.
• Drill-down: download the observation artifact, then run homeboy runs import <dir>, homeboy runs list, and homeboy runs findings <run-id>.
• Artifacts are attached to the workflow run: https://github.com/Extra-Chill/data-machine/actions/runs/28970015680

Audit

✅ review audit — passed

• audit — 43 finding(s)
• Total: 43 finding(s)

Deep dive: homeboy review audit data-machine --changed-since ad09ae9

Artifacts and drill-down

• CI results artifact: homeboy-ci-results-data-machine-review-audit-quality-Linux-node24 contains immediate command JSON for this action invocation.
• Observation artifact: homeboy-observations-data-machine-review-audit-quality-Linux-node24 contains exported Homeboy run history for deeper queries.
• Drill-down: download the observation artifact, then run homeboy runs import <dir>, homeboy runs list, and homeboy runs findings <run-id>.
• Artifacts are attached to the workflow run: https://github.com/Extra-Chill/data-machine/actions/runs/28970015680

Tooling versions

• Homeboy CLI: homeboy 0.281.20+123145ee082b+dcdd8a98
• Extension: wordpress from https://github.com/Extra-Chill/homeboy-extensions
• Extension revision: f89b4297
• Action: unknown@unknown