[CI] Docs tier-drift's path filter watches the templates but not docs/guide, so a change to the generated output alone skips the only gate that checks it

## Summary

`docs/guide/*.md` is generated from `docs/_pipeline/templates/*.md.dt`. The only gate that detects drift between them is the **`Docs tier-drift`** CI job — and its path filter watches the **templates**, not the **generated output**.

**A change that touches only `docs/guide/*.md` and leaves the template untouched sets `docs-templates=false`, skips the job, and ships with no gate at all.**

```
OBSERVED_AT 2026-08-01T13:45:35Z main = 1ed2644e

.github/workflows/ci.yml:71
if echo "$files" | grep -qE '^(docs/_pipeline/templates/|docs/_pipeline/apps/|src/Reactor\.Cli/Docs/)'; then
docs-templates=true

'docs/_pipeline' in the filter block : True
'docs/guide' in the filter block : FALSE <- the artifact it protects
```

## No unit-test gate compensates

The other two generated artifacts in this repo are gated by tests that run in the `Unit Tests` leg, which executes on every non-Markdown change:

| artifact | gate | runs when |
|---|---|---|
| `samples/ReactorGallery/reactor-search-index.json` | `SearchIndexGeneratorTests.Index_IsUpToDate` | every non-md change |
| `skills/reactor.api.txt` (+ plugins copy) | `ApiIndexGeneratorTests` | every non-md change |
| **`docs/guide/*.md`** | **`Docs tier-drift` CI job only** | **only when a template/app/CLI path changes** |

Searched every test referencing `md.dt` — `CrossLinkLintTests`, `DiagramTests`, `TierLintTests`, `TierLintOrchestratorTests`, `FlexPanelCssBehaviorFixtures`, `VersionSingleSourceTests`. **The only one touching both `docs/guide` and a compile step is `VersionSingleSourceTests`, and it is version-only** — seven tests about version literals/tokens, `Assert.Equal` occurrences: **0**.

## Why this is reachable, not theoretical

Found while verifying a merge. `git merge-tree` on two open PRs reports:

```
CONFLICT (content): ... 5 files ...
Auto-merging docs/_pipeline/templates/data-system.md.dt
Auto-merging docs/guide/data-system.md <- generated output, SILENT
Auto-merging tests/.../DataGridEditFixtures.cs
```

**Git merges the template and its generated output as two unrelated text files.** It can interleave two generated outputs into a file the generator would never emit, and **no conflict marker appears on either.**

In *that particular* merge the template also changes, so the gate would fire. **That is incidental.** Any of these skips it:

- a merge where only one side changed the template, so only the output auto-merges
- a hand-edit of `docs/guide/*.md` (forbidden by `AGENTS.md` — and this gate is what enforces it)
- a revert or cherry-pick touching only the output

> **The absence of a conflict marker is not evidence that regeneration can be skipped — and here nothing downstream will catch it either.**

## Suggested fix

Add `docs/guide/` to the `docs-templates` filter at `ci.yml:71`:

```bash
if echo "$files" | grep -qE '^(docs/_pipeline/templates/|docs/_pipeline/apps/|src/Reactor\.Cli/Docs/|docs/guide/)'; then
```

**The job already does the right comparison** (`docs compile --no-screenshots --ci` then fails with *"docs/guide is out of date with the doc templates"* at `:670`) — it simply is not invited to run when only its subject changes.

**Non-vacuity check for the fix:** modify one byte of a committed `docs/guide/*.md` without touching any template, and confirm `Docs tier-drift` runs *and* fails. Under the current filter it does not run at all.

---

Surfaced by the #1010 session's observation that `merge-tree`'s auto-merge lines matter more than its conflict list; filter and gate coverage verified independently here.

Start in .github/workflows/ci.yml around line 71, where the docs-templates path filter is set, then inspect the Docs tier-drift job around line 670 and its docs compile --no-screenshots --ci check. Update the filter coverage and verify that changing only a committed docs/guide/*.md file causes the job to run and report drift.

Written by the indexing model from the issue text.