docs: document incremental builds and the session design

README: the build-session workflow (auto-recording, --resume,
'pkh deb list', --keep, prune retention) under a new section.
plans/: the design spec with the decisions taken along the way
(opt-in resume, --keep for the iteration loop, /var/tmp/pkh/
sessions, Ctrl+C keeping the session) and the implementation notes
recording the as-built deviations.

README.md

@@ -111,6 +111,28 @@ pkh put --ppa user/hello_xxx
 git push xxx user-fork
 ```
 
+### Incremental builds (build sessions)
+
+Every `pkh deb` build records a **session** under `/var/tmp/pkh/sessions`:
+the bootstrapped chroot, the installed build dependencies and the build
+artifacts of the staged tree. When a build fails (or is interrupted), the
+session is kept and can be resumed:
+
+```
+pkh deb                 # fails after 25 minutes
+pkh deb --resume        # reuses the chroot, build deps and objects;
+                        # only what changed is recompiled
+pkh deb list            # the sessions of this tree, with their ids
+pkh deb --resume <id>   # resume a specific session
+pkh deb --keep          # keep the session even after a successful build
+                        # (iterate: edit, `pkh deb --resume --keep`, ...)
+pkh prune               # garbage-collect old sessions (7-day retention)
+```
+
+A plain `pkh deb` never reuses a session — everything is rechecked from
+scratch — and it replaces the session of its target. `pkh deb --resume`
+refuses to adopt a session built for a different series/architecture.
+
 ## Future improvement ideas
 
 - pull: try to fetch the correct git branch for series on Debian