Add grep verification step to fragment-anchor guidance in boost AGENTS.md

What happened

On PR #4534, the code agent created 4 markdown links with display text "Decision 1" pointing at design.md without fragment anchors. Human reviewer mareklibra pushed commit c36fcd4 to add the anchors manually. The retro agent filed issue #4539, and PR #4540 added a "Fragment anchors for heading references" subsection to workspaces/boost/AGENTS.md (lines 285-298). The new guidance tells agents to read the target file, find the matching heading, and convert it to a GitHub-style fragment — but unlike the adjacent "Relative markdown links" subsection (lines 266-283), it does not include a concrete shell-command verification step.

What could go better

The sibling "Relative markdown links" subsection includes an explicit ls <relative-path-from-link> verification command (line 282). The fragment-anchor subsection has no analogous verification. This matters because PR #4534 demonstrated that the code agent can have verification guidance available yet still skip it — on that PR the agent ran a cosmetic bash comment instead of the prescribed ls check, leading to a broken path that the review agent caught but that cost $4.90 in fix + re-review. Without a concrete verification command for fragment anchors, the same pattern could recur: the agent may construct a fragment from memory or assumption rather than confirming the heading actually exists in the target file. Confidence: moderate. The guidance's instruction to "Read the target file, find the matching heading" is clear in intent, but the PR #4534 precedent suggests that explicit shell-command verification is more reliably followed than prose instructions.

Proposed change

Add a verification block to the "Fragment anchors for heading references" subsection in workspaces/boost/AGENTS.md (after line 298, before the ## Build & verify section). The block should mirror the structure of the relative-link verification at lines 278-283:

Always verify the heading exists in the target file:

```bash
# From the repo root or the directory containing the target file:
grep -n '### Decision 1' <path-to-target-file>

This makes the verification step concrete and runnable, consistent with the adjacent subsection's pattern. The grep command confirms the heading text exists before the agent converts it to a fragment anchor.

## Validation criteria

The next 3 code agent PRs in the boost workspace that add cross-reference links with heading-specific display text should show the agent running a grep or similar command to verify the heading exists in the target file before constructing the fragment anchor. The review agent should flag any heading-referencing link whose fragment does not match an actual heading in the target document.

---
_Generated by retro agent from https://github.com/redhat-developer/rhdh-plugins/pull/4540_

Open workspaces/boost/AGENTS.md and compare the “Fragment anchors for heading references” subsection with the adjacent “Relative markdown links” verification block. Add the requested grep-based verification example before “Build & verify”; done means the guidance includes a concrete command confirming the referenced heading exists.