feat(dsh): adapt to DSH 0.1.5-alpha.2

Baseline moves to DSH 0.1.5-alpha.2 only (peer floor ^0.1.5-alpha.2,
CI pin, manifest engines); alpha.1 users stay on v0.19.0-alpha.0.

Host-contract changes handled:
- the native guide entry lost its `description` field (entries are
  icon+title capsules now): `TabDescriptor.description` and the six
  `guideDesc*` keys plus the `nativeGuideDesc` fallback are removed.
- the file address grammar changed: `fileAddressFor` always returns a
  session-scoped address (absolute paths keep their leading `/`),
  `parseFileAddress` is prefix-based and strips `?`/`#`. The plugin's
  purity-gated mirror follows.
- global main panels replaced the `conversation` root slot with the
  keyed `main` slot (`main.conversation`): the bottom workbench's
  center-column locator now accepts both keys and skips
  `display: contents` slot hosts.

Fixes a pre-existing fill bug the alpha.2 probe made obvious: the native
tab body host is a block scroller with a definite height, so tab roots
that only declared `flex: 1` collapsed to content height and the side
chat composer drifted off the pane bottom. The native adapter now wraps
every tab body in a `height: 100%` column flex host.

Also promotes the dependencies `dsh-client-ui-primitives@0.1.5-alpha.2`
dropped from its manifest (anser / shiki / mdast-util-* / micromark-* /
katex / @shikijs/langs) and the newly unmet `dsh-session-persistence`
peer into devDependencies.

.github/workflows/ci.yml

@@ -131,10 +131,10 @@ jobs:
       # Pin the DSH CLI the lane mounts into; the profile's bundle stack
       # (dsh-base / dsh-web-app) resolves from this installation. Keep in
       # step with the plugin's @deepseek-ai/* peer range when bumping.
-      # DSH 0.1.5-alpha.1 is published under the `alpha` dist-tag; the lanes
+      # DSH 0.1.5-alpha.2 is published under the `alpha` dist-tag; the lanes
       # speak the slash-only RPC dialect (tests/e2e/host-protocol.ts).
       - name: Install dsh CLI (pinned)
-        run: npm install -g @deepseek-ai/dsh@0.1.5-alpha.1
+        run: npm install -g @deepseek-ai/dsh@0.1.5-alpha.2
 
       # lib/ is gitignored; the tarball is produced from this build.
       - name: Build and pack the npm tarball