chore(desktop): hide custom instructions from task timeline

[tatoalo]

Problem

Saved custom instructions are agent context and should not be exposed in the task activity timeline.

Changes

• Strip injected custom-instruction blocks from timeline message previews using the existing transcript parser.
• Add regression coverage for the timeline display.

[trunk-io]

😎 Merged successfully - details.

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 7a2e2c5.}

[greptile-apps]

Prompt To Fix All With AI

### Issue 1
packages/ui/src/features/canvas/components/ActivityTimeline.tsx:86-90
**Literal instruction tags are removed**

When a user intentionally includes a complete `<user_custom_instructions>...</user_custom_instructions>` example, the new parsing step treats it as injected metadata and removes it, causing the timeline preview to omit legitimate user-authored content.

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

_{Reviews (1): Last reviewed commit: 81463b5 | Re-trigger Greptile}

[github-actions]

Small, test-covered UI display change reusing an already-shipped stripping utility; the unresolved Greptile comment flags a pre-existing edge-case limitation of that shared helper (not a new regression), is cosmetic only, and a human reviewer approved the current head.

• puemos reviewed the current head.

Gate mechanics and policy version

Gate		Result
prerequisites	✓	all clear
deny-list	✓	no deny categories matched
size	✓	8L, 1F substantive, 26L/2F incl. docs/generated/snapshots — within ceiling
tier	✓	T1-agent / T1b-small (26L, 2F, single-area, chore)
stamphog 2.0.0b3		.stamphog/policy.yml @ 32c0189 · reviewed head 81e2f27

New commits pushed (delta classified non_trivial_delta) — stamphog approval dismissed; re-review running automatically.

[github-actions]

Small, well-tested UI fix to timeline display logic (no risky territory touched); the reviewer-flagged false-positive stripping issue was fixed with a tighter regex and covered by new regression tests, with no unresolved concerns.

• 👍 on the PR from greptile-apps[bot].

Gate mechanics and policy version

Gate		Result
prerequisites	✓	all clear
deny-list	✓	no deny categories matched
size	✓	14L, 2F substantive, 63L/5F incl. docs/generated/snapshots — within ceiling
tier	✓	T1-agent / T1b-small (63L, 5F, single-area, chore)
stamphog 2.0.0b3		.stamphog/policy.yml @ aa6b6ca · reviewed head 7a2e2c5