fix(cli): keep unrelated AGENTS.md content intact on remove

[L4XB]

Summary

ctx7 remove for the append-style rule files (AGENTS.md, GEMINI.md) cut the <!-- context7 --> section and then ran .replace(/\n{3,}/g, "\n\n").replace(/^\n+/, "") on the whole file. That silently rewrote content the CLI never added: two blank lines inside a fenced code block were collapsed, and the spacing between the user's own sections changed.

This PR normalizes only the seam left by the removed section (at most one blank line between the surrounding content, a single trailing newline) and leaves everything else byte for byte as it was. The section lookup itself is unchanged.

Before / after for

# Project rules

```text
line a


line b
```


## Team notes

<!-- context7 -->
…
<!-- context7 -->

## Tail section

• before this PR: the blank lines inside the code block and before ## Team notes are collapsed
• after this PR: only the Context7 section is gone, the rest is identical

Tests

Two cases added to packages/cli/src/__tests__/remove.test.ts: the file above is compared with toBe against the expected byte-identical result, and a trailing section removal keeps the preceding content (including trailing spaces on the user's lines).

pnpm --filter ./packages/cli test    # 19 files, 370 tests
pnpm lint:check / typecheck          # clean

Changeset: ctx7 patch.