🤖 feat: optional flat chat list for the sidebar with project badges - #3994

- workspaceService flat-reorder mock (#3994) sets getSessionDir on
  Partial<Config>, which the split Config (#4017) no longer has; the test
  only needs sessionsDir
- ProjectSidebar flat-list mocks (#3994) lack archivingWorkspaceIds (#4068)
- MCP prompt snapshot test asserts the cancelSignal argument this PR removed

## Summary

Settings > General groups its controls under headings (Appearance,
Sidebar, Transcript, Terminal, Archiving, Editor & debugging, Projects),
but only whitespace separated the groups, so the boundary between one
group's last row and the next heading was hard to see. This adds a thin
horizontal rule between the groups.

## Implementation

Each group wrapper after the first gets `border-border-light border-t
pt-6`: a 1px rule in the theme border color (the same token
`divide-border-light` uses elsewhere in Settings), with the existing
`space-y-6` gap above the line and `pt-6` below it so the heading is not
glued to the rule. No rule above Appearance. Six className edits in
`GeneralSection.tsx`, no new components or CSS.

## Validation

- Storybook `GeneralSection` story at 1100px and 390px: computed styles
show `0px` border on the Appearance wrapper and `1px rgb(38,38,38)` +
`24px` padding-top on the other six; no horizontal overflow.
- Remote dogfood UAT (Coder Agents) on this exact commit: six dividers
with even spacing in dark and light themes at desktop and 390px,
placement holds when the Terminal group grows/shrinks (Terminal Badge
on/off), other tabs unchanged.
- UAT also surfaced a pre-existing issue, not touched here: at 390px the
Terminal Font input (fixed `w-80`) overflows the settings column;
reproduced on the base commit.

> Note: the first CI run inherited two failures from a then-red `main`
(coder#3994's stale `getSessionDir` mock and five `ProjectSidebar flat chat
list` renders missing `archivingWorkspaceIds`); coder#4077 fixed both on
`main`, which is merged in here.

---

_Generated with `xum` • Model: `anthropic:claude-fable-5-1` • Thinking:
`xhigh` • Cost: `$19.46`_

<!-- mux-attribution: model=anthropic:claude-fable-5-1 thinking=xhigh
costs=19.46 -->

## Summary

Version bump for the v0.28.5 patch release. Headline changes since
v0.28.4: remote server connections in the desktop app (coder#4101),
self-updating `xum server` under a restart supervisor (coder#4083, coder#4127),
first-class GPT-6 Astra and Astra Pro support including Codex OAuth
routing (coder#4064, coder#4094, coder#4106, coder#4124), token-budget context window
rollovers (coder#4097), the workspace remembering model and mode on send
(coder#3968), in-place plugin updates (coder#4164), the optional flat sidebar chat
list (coder#3994), and copying selected chat text as Markdown (coder#4170). It
also carries a long run of streaming, compaction, and task-lifecycle
fixes (reconnect streaming coder#4123, message edits during active streams
coder#4153, Codex OAuth prompt-cache routing coder#4159, compaction/history
fencing coder#4133 through coder#4148, task lock ordering coder#4161) plus the Effect
Wave 4 runtime refactors and deslop passes 1 through 3.

## Implementation

Bumped with `node ./scripts/set-package-version.js 0.28.5` so the root
`package.json` and the legacy `packages/mux-compat` forwarding package
stay version-locked. `src/common/compat/productIdentity.test.ts` passes
locally (8/8).

After this PR merges, the `v0.28.5` tag will be applied to the squash
commit and the GitHub Release published to trigger the
desktop/npm/docker pipelines.

---

_Generated with `xum` • Model: `anthropic:claude-fable-5-1` • Thinking:
`xhigh` • Cost: `$1.64`_

<!-- mux-attribution: model=anthropic:claude-fable-5-1 thinking=xhigh
costs=1.64 -->