Artifact: PR Description Scoped to One Concern

ex-11 · exercises co-06 · a pull-request description a reviewer can summarize in one sentence.

A description scoped to one concern states what changed, why, and how it was checked -- and nothing else. A reviewer reading it should be able to restate the PR's purpose in a single sentence before ever opening the diff.

## What
 
Adds `Client.refresh_token(old: str) -> str`, so an expiring session token can be renewed without a
full re-login.
 
## Why
 
Support tickets show ~4% of daily active sessions hit a hard logout mid-checkout when the access
token expires -- refreshing in place removes that failure mode entirely.
 
## How this was checked
 
- Added `test_refresh_token_returns_new_token` and `test_refresh_token_rejects_expired_refresh_token`
  (both pass locally: `pytest -k refresh_token`).
- Manually verified against the staging auth service: a token refreshed 30 seconds before expiry
  keeps the session alive with no re-login prompt.
 
## Not in scope
 
Refresh-token rotation policy (tracked separately: TECH-DEBT-041) is intentionally out of scope --
this PR only adds the refresh call itself.

Verify: a reviewer can restate this PR's purpose in one sentence ("lets an expiring session token refresh in place instead of forcing a re-login") after reading only the ## What/## Why sections -- satisfying co-06's single-concern rule. The ## Not in scope section actively prevents scope creep by naming the adjacent work it deliberately excludes.

Key takeaway: ## What / ## Why / ## How this was checked / ## Not in scope is a small, repeatable template that forces a description toward one concern, one sentence, every time.

Why It Matters: a PR description that lists five unrelated changes forces the reviewer to reconstruct the actual scope from the diff itself -- exactly the cognitive tax co-06's "one PR, one concern" rule exists to remove. ## Not in scope is the cheapest line in the template and the one most PRs skip, even though it is what stops "while I was in there" scope creep before it starts.