docs: dsh 0.1.5-rc.1: Session log format v3 with new event vocabulary

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dsh 0.1.5-rc.1 lands a structural change to the Session log — the durable,
on-disk record of every agent interaction — so this release warrants a
human review pass. The persisted event vocabulary changed (assistant/chunk
is gone; system/message and assistant/attempt are new; SurfaceOp.replace
renamed its payload fields), the format writer is now a versioned generation
chain (v0/v1/v2 migrate forward to the current writer) instead of the old
"pre-release, pinned at 0, no compatibility implied" stance, and the docs add
a dedicated version-and-release-status page plus a cookbook for adding the next
edge. Anyone with sessions written by an older dsh will hit translation on open,
and anything parsing the JSONL directly (or merging event types from an
out-of-tree plugin) will break. The npm release also moved, which is itself the
signal this shipped.

Highlights, most interesting first:

• Session log format is now versioned and migrated: current writers stamp
  SESSION_FORMAT_VERSION (= 3), and released historical formats v0/v1/v2 are
  translated to the current vocabulary on stat/list/open. Historical
  migration refuses unknown event types even when marked ignorable: true;
  current-format restoration still honors them. Old logs are read without
  mutating the source file — a write open publishes the up-converted generation.
• New session-format-status.md is now the single authority for "what format
  is current / next / released" (checkout writer in code, latestReleasedVersion
  in a release record, evidence tag dsh-v0.1.5-alpha.1). Docs elsewhere link
  to it instead of restating a number — worth a look because it is the new
  contract for anyone reasoning about log compatibility.
• Event vocabulary broke: assistant/chunk removed in favor of per-attempt
  compact stream records embedded in assistant/message and a new
  assistant/attempt event for failed/retried/cancelled attempts that produce
  no surface message; system/message added as surface node 0 (the system
  prompt is no longer part of request/header); SurfaceOp replace now uses
  startSeq/endSeq; session/end-seed carries { inherited?: true }.
• SessionHandle.read() now returns SessionHandleReadResult ({ eventState, events }) rather than a bare array — an interface change for any out-of-tree
  persistence backend. RestoredSessionOptions.seedSource became eventState
  (detached | shared-frozen).
• request/header is stricter: a system field is forbidden, tools: [] and
  adapterDefaults: {} must be omitted; noncanonical headers are rejected at
  seed/append/load instead of normalized.
• npm moved to latest 0.1.5-rc.1 / alpha 0.1.5-alpha.2 (from latest 0.1.1-rc.2), giving the new Session-format obligations a real published
  release.
• New str_replace_editor opt-in for the Python SDK repo: sdk-minimal no
  longer ships the editor (nor the filesystem provider); users pass a
  cordis.patch.yml via patches= or $DSH_HOME/profiles/... to add it back.
• New web-client subsystem pages: client-resources.md (address-based
  dsh-resource:// protocol + useResource hook) and sidebar-right.md
  (per-Session docking surface, tab-type registry, navigation service).
• New user guide network-proxy.md: HTTP(S)_PROXY support, NO_PROXY
  exclusions, credentials-in-URL, and the paths that stay direct (with a
  warning that a project .env cannot set proxy vars).
• Attachments widened from image-only to generic files (ctx.fileUploads,
  admitPromptContent, saveFile), and feedback moved into the canonical log
  as feedback/message-put/feedback/message-delete log-only events.
• Smaller additions: new present tool (deliverables/presented), a Windows
  and WSL 2 development section, provider reasoning-effort configuration
  (reasoningEfforts, compat.thinkingFormat: deepseek), and a workspace-files
  API/service.