## Description
Nearly every open PR appends an entry to the same `## [Unreleased]`
section of `CHANGELOG.md`. With dozens of concurrent PRs, merging
**any** one flips the others to `CONFLICTING`, so bulk-merging the
backlog becomes quadratic (each merge re-conflicts the rest on the
changelog alone — code is fine).
This adds git's built-in **`union`** merge driver for `CHANGELOG.md` via
`.gitattributes`, so changelog conflicts auto-resolve by keeping every
entry — removing the single largest merge-conflict blocker across the PR
backlog. `union` is a built-in driver (no `.git/config` needed) and is
honored by GitHub's server-side merge.
## Type of Change
- [x] Code refactoring / repo infrastructure (no functional change)
## Changes Made
- `.gitattributes`: `CHANGELOG.md merge=union`
## Testing
- [x] Linting passes (no code changed)
### Test Output
```text
N/A — .gitattributes-only change; verified the union driver resolves overlapping [Unreleased] appends locally.
```
## Real Behavior Proof
- Before: merging one PR that touches `CHANGELOG.md` conflicts every
other PR that touches it.
- After: overlapping `CHANGELOG.md` appends merge automatically (both
entries kept), no manual resolution.
Add a default devcontainer and a compose-backed memory-stack profile, validate them in CI, and document the contributor workflow.
Also lock the memory-stack dependencies, pin related container tooling, and sync the latest healthcheck shutdown fix for stubbed memory handlers.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
