fix: relax CodeRabbit rebase guideline to not check commit distance from main (#7475)

Signed-off-by: Keiven Chang <keivenchang@users.noreply.github.com>

.ai/pr-workflow-guidelines.md

-# PR Workflow Guidelines
-
-Conventions for keeping pull requests healthy and reviewable.
-
-## Keep Your Branch Close to Main
-
-Be aware of PRs targeting `main` that are more than ~25 commits behind.
-Stacked PRs targeting another branch are exempt.
-
-### 1. Slower CI builds
-
-CI builds start from a base image matching a recent `main` commit. The closer
-your branch is to `main`, the more cache hits you get. We use BuildKit with
-remote cache (`--cache-from`), but BuildKit layers are still content-addressed
--- if a `COPY` brings in source that changed since your branch point, that
-layer and everything downstream rebuilds regardless. When your branch is far
-behind:
-
-- **Docker layer cache misses**: Changed source or Dockerfile instructions
-  since your branch point invalidate cached layers, forcing rebuilds
-  downstream.
-- **Rust compilation cache misses**: More crate sources differ from what's
-  cached, so more crates recompile from scratch.
-- **Dependency drift**: `Cargo.lock` and `requirements.txt` evolve on `main`.
-  Stale branches pull older versions that aren't cached, triggering fresh
-  downloads and builds.
-
-A branch too far behind `main` can trigger a near-cold build that takes
-45-60 minutes. A branch close to `main` reuses most of the cache and builds
-in a fraction of that time.
-
-### 2. Reviewer burden
-
-Merge commits from main pollute the diff with unrelated changes. Rebasing
-produces a clean, linear history where every commit is the PR author's.
-
-```bash
-# BAD -- merge pulls in unrelated files; reviewer has to filter them out
-git merge main
-
-# GOOD -- clean diff showing only your work
-git fetch origin && git rebase origin/main
-```
-
-### Guidance
-
-- **Rebase when you are more than ~25 commits behind main.**
-- **Prefer `rebase` over `merge`** for linear history. Force-push after
-  rebasing (`git push --force-with-lease`).
-- **Before requesting review**, check your distance from main:
-  ```bash
-  git fetch origin
-  git rev-list --count HEAD..origin/main
-  ```
-  More than 25? Rebase first.
-- **If CI is slow**, check your base commit age before debugging other causes.
-- **Stacked PRs** are exempt. If a PR targets another branch in a stack,
-  distance from `main` is expected and not a problem until the stack lands.

.coderabbit.yaml

@@ -28,11 +28,6 @@ reviews:
     unit_tests:
       enabled: false
   path_instructions:
-    - path: "**"
-      instructions: |
-        Flag PRs more than ~25 commits behind main. Recommend rebasing.
-        Exception: stacked PRs targeting another branch are exempt.
-        See .ai/pr-workflow-guidelines.md.
     - path: "**/*.py"
       instructions: "Review against .ai/python-guidelines.md"
     - path: "**/tests/**/*.py"