fix(slack): enable preview streaming in flat DMs (replyToMode: off) (#76330)

Summary:
- The PR enables Slack draft preview streaming for flat DMs in all non-off modes, updates Slack streaming tests/docs/config metadata/changelog, and refreshes small guard baselines.
- Reproducibility: yes. source-reproducible on current main: the helper returns false for `mode: "partial"` in ...  current test asserts that disabled path. I did not run tests because this review was explicitly read-only.

Automerge notes:
- Ran the ClawSweeper repair loop before final review.
- Included post-review commit in the final squash: fix(slack): enable preview streaming in flat DMs (replyToMode: off)
- Included post-review commit in the final squash: fix(clawsweeper): address review for automerge-openclaw-openclaw-5654…

Validation:
- ClawSweeper review passed for head 52e5d74ef9.
- Required merge gates passed before the squash merge.

Prepared head SHA: 52e5d74ef9
Review: https://github.com/openclaw/openclaw/pull/76330#issuecomment-4365017023

Co-authored-by: clawsweeper <274271284+clawsweeper[bot]@users.noreply.github.com>
Co-authored-by: bob <637186+HangGlidersRule@users.noreply.github.com>

src/plugins/contracts/extension-runtime-dependencies.contract.test.ts

@@ -25,6 +25,11 @@ const INDIRECT_RUNTIME_DEPENDENCIES = new Map<string, Set<string>>([
     // Baileys loads jimp as an optional peer when it needs media thumbnails.
     new Set(["jimp"]),
   ],
+  [
+    "extensions/tlon",
+    // The Tlon plugin manifest exposes the bundled skill from this package path.
+    new Set(["@tloncorp/tlon-skill"]),
+  ],
 ]);
 
 type PackageManifest = {