feat(engine): add datamachine_engine_snapshot filter at job init

[chubes4]

Adds a one-line extension point to `RunFlowAbility::execute()` so downstream plugins can enrich the engine snapshot at job initialization without forking DM or building parallel persistence machinery.

Motivation

DM core has no way for an external plugin to inject runtime context into a job's `engine_data` at the moment the job is created. Today the only public path is the `initial_data` input on `run-flow` — which is fine when the caller knows the context, but useless for cases where the context lives in a third plugin that the caller doesn't know about.

Concrete driver: Data Machine Code already tracks every workspace and worktree's identity (`WorktreeContextInjector`), but that knowledge never reaches AI directives, abilities, or tool calls during a job because there's no point in the run-flow pipeline where DMC can stamp "hey, this job is bound to `extrachill-artist-platform@docs/agent-run-123`" into `engine_data`. See Extra-Chill/data-machine-code#423.

What this adds

One `apply_filters()` call at the right spot:

```php
$filtered_snapshot = apply_filters( 'datamachine_engine_snapshot', $engine_snapshot, $job_id, $flow, $pipeline );
if ( is_array( $filtered_snapshot ) ) {
$engine_snapshot = $filtered_snapshot;
}
```

Fires immediately before `datamachine_set_engine_data()`. Filters receive the assembled snapshot, the job ID, and the source flow + pipeline rows. Returning a modified array enriches the snapshot. Returning a non-array silently preserves the prior snapshot so a misbehaving filter cannot corrupt `engine_data`.

Why this shape

Alternative considered	Why rejected
New schema / table for "runtime context"	Massive overkill for what's a one-time-per-job context attachment
New CLI surface	No reason — the filter is invisible to operators
New ability	Same job/snapshot context, abilities can't observe the in-flight assembly
Action hook only (no return value)	Filters compose better — multiple plugins can stack enrichments
Filter after persist	Too late — `engine_data` is already the source of truth by then

The filter pattern matches existing DM conventions (`datamachine_directives`, `datamachine_memory_files`, `datamachine_agent_modes`, `datamachine_agent_mode_{slug}`). One line, fully backward-compatible — DM runs unchanged with no callbacks registered.

Downstream consumers

• Data Machine Code — projects `active_workspace` (repo, handle, branch, path, origin metadata) into the snapshot when callers pass a workspace handle via `initial_data.active_workspace.handle`. See Expose active workspace identity (repo, handle, branch) to AI execution context data-machine-code#423 + its companion PR.
• Future: any plugin that has per-run context it wants visible to AI directives — security scoping, audit attribution, environment tagging, etc.

Smoke testing

`php -l` clean. Pattern is structurally identical to the existing `datamachine_directives` filter loop already in DM; no new code paths.

End-to-end smoke (a filter callback that mutates `engine_data` and an AI step reading it back) is exercised by the companion DMC PR's projector against the artist-platform docs-agent pilot.

Scope

One file, one filter, 24 net lines (mostly the docblock). No tests added in this PR because the change is purely a passthrough; the actual behavior under test belongs to the downstream consumer.

Refs Extra-Chill/data-machine-code#423
Refs Extra-Chill/extrachill-docs#31

[homeboy-ci]

Homeboy Results — data-machine

Lint

✅ lint — passed

ℹ️ Full options: homeboy docs commands/lint
Deep dive: homeboy lint data-machine --changed-since 3f8abcb

Test

✅ test — passed

ℹ️ No impacted tests found for --changed-since 3f8abcb
ℹ️ Run full suite if needed: homeboy test data-machine
Deep dive: homeboy test data-machine --changed-since 3f8abcb

Audit

✅ audit — passed

• requested_detectors — 6 finding(s)
• Total: 6 finding(s)

Deep dive: homeboy audit data-machine --changed-since 3f8abcb

Tooling versions

• Homeboy CLI: homeboy 0.182.0+1f8c0496
• Extension: wordpress from https://github.com/Extra-Chill/homeboy-extensions
• Extension revision: dd47f26a
• Action: unknown@unknown