feat(session): opt-in packed chunk rows in the JSONL log

Providers stream token-sized deltas, so a session log stores hundreds of
near-identical assistant/chunk lines whose JSON envelopes dwarf their
payloads (~56x measured on a real DeepSeek session, 73% of file bytes).

Add a lossless storage codec to dsh-session: packChunkRuns() folds each
run of >=3 consecutive same-block delta chunks into one storage row --
text-chunks / reasoning-chunks / tool-call-chunks, bare slash-less tags
like the header line's 'session' so rows cannot be confused with session
events -- and decodeStorageRecord() expands rows back to the exact
original events (seq0/time0 + dt gap array reconstruct every member's
seq/time; tool-call rows carry the run-constant id/name). The encoder
whitelists exact shapes and stores anything unrecognized verbatim; the
decoder validates row-tagged values and fails loud on malformation.

The JSONL backend gains a packChunks config (default false). Writing
packs only when enabled -- default-off output stays byte-identical to
the previous layout, so snapshot goldens are untouched. Reading is
layout-blind: scanLog always decodes rows and now checks seq contiguity
with a cursor instead of the line index, so packed, unpacked, and mixed
files all load identically. Fixture readers (llm-replay parseSessionLog,
acp-snapshot normalizeSessionLog) share the codec; the normalizer zeroes
a row's time0/dt exactly like an event's time. The two demo bundles
plumb packChunks from cordis.yml to the backend.

Measured on a real coding session: 105 KB -> 42 KB (-60%), 475 lines ->
74, with reasoning/tool-call heavy sessions saving the most. Covered by
example + fast-check round-trip codec tests, backend packed/mixed/torn-
tail specs, and an end-to-end demo run loading a packed log through a
default-config backend.

packages/examples/acp-demo/src/index.ts

@@ -39,6 +39,8 @@ export interface Config {
   tools?: ToolsConfig
   /** Directory the JSONL session backend writes under. Defaults to `./.sessions`. */
   persistenceRoot?: string
+  /** Write delta-chunk runs as packed storage rows (the JSONL backend's `packChunks`). Defaults to `false`. */
+  packChunks?: boolean
   /** Skill registry, local-provider, and model-facing consumer config forwarded to agent-spine-demo. */
   skills?: agentCore.SkillConfig
 }
@@ -57,6 +59,7 @@ export const Config: z<Config> = z.object({
   // TODO(single-default-literal): share this schema default and the defensive
   // apply() fallback through one named constant while retaining both boundaries.
   persistenceRoot: z.string().default('./.sessions'),
+  packChunks: z.boolean().default(false),
   skills: agentCore.SkillConfigSchema,
 })
 /* jscpd:ignore-end */
@@ -76,6 +79,9 @@ export function apply(ctx: Context, config: Config): void {
     ...config.skills !== undefined ? { skills: config.skills } : {},
   })
   ctx.plugin(UserInteractionService)
-  ctx.plugin(SessionPersistenceJsonl, { root: config.persistenceRoot ?? './.sessions' })
+  ctx.plugin(SessionPersistenceJsonl, {
+    root: config.persistenceRoot ?? './.sessions',
+    ...config.packChunks !== undefined ? { packChunks: config.packChunks } : {},
+  })
   ctx.plugin(acp, { model: config.model })
 }