docs: align README and prompt docs with the real runtime semantics

README described target-state features that the previous migration
left half-implemented. Now that the runtime actually matches the
promise, the docs can stop overselling: agent messaging is
described as the real live-steer + inbox flow, approvals as the
beforeToolCall gate that actually blocks destructive tools,
sessions as the transformContext-driven compaction plus rotation.
The architecture bullet list grows to name the seams that landed
across this migration — the pi contract wiring, the approval gate,
the workspace lock model, and the stripped startup mutations — so
the README reads like documentation rather than roadmap.

The filesystem prompt doc (`files.en.md`) adds the missing
preamble: every bash/write/edit succeeds into work_products
automatically, and the API endpoints below are only needed for
non-filesystem deliverables. Agents were routinely posting
duplicate work products because nothing told them the projection
already ran.

src/main/prompt-builder/api-docs/files.en.md

@@ -1,3 +1,8 @@
+### Filesystem artifacts (auto-registered)
+Every successful `write`, `edit`, or `bash` tool call is projected into the task's work_products automatically — you do NOT need to POST /api/work-products for files you create in the workspace. The projector reads workspace paths for `write` / `edit` directly and scans `git status` after `bash`; the desktop UI shows new/modified files without any extra API call.
+
+Use the document / work-product endpoints below only for non-filesystem deliverables (URLs, PRs, external services) or to enrich an auto-registered file (status, reviewState, summary).
+
 ### Create a Document
 POST /api/companies/{{companyId}}/documents
 Body: {