docs: split context into layered, agent-oriented files

Replace the single file-structure.md with a stratified layout designed
for AI/agent skill consumption: tables and concrete identifiers over
prose, files loaded on demand, content separated by churn rate.

Layers:
- architecture/  stable structural reference (overview, source-map,
                 test-map, data-flow)
- patterns/      memory, commands, autocomplete, events, xmpp,
                 encryption, ui, plugins
- testing/       unit-tests, stubs, functional-tests, bench
- build/         local, docker, ci
- playbooks/     add-command, add-test, add-autocomplete,
                 add-event-handler, add-encryption
- gotchas.md     append-only dated entries (seven seed entries)
- wip/           branch-specific notes; deleted on merge to master

Stable layers describe cproof on master only. In-flight feature
branches (currently feat/ai) get a single file under wip/.

INDEX.md is the entry map with churn labels; SKILL.md is the
always-loaded skill hint pointing to it.

build/local.md

@@ -0,0 +1,68 @@
+# Local build
+
+Reference for `autogen.sh` / `configure` / `make` flags. **Do not run these
+on the host as a default workflow** — see `build/docker.md`. This file
+documents what the canonical Docker build does internally.
+
+## Toolchain
+
+- autotools: `autoconf`, `automake`, `libtool`, `autoconf-archive`.
+- C compiler: `gcc` (CI) or `clang`.
+- GLib 2, ncursesw, OpenSSL, libstrophe, libreadline.
+- Optional: libcmocka (tests), libsignal-protocol-c (OMEMO), libotr5 (OTR),
+  gpgme (PGP), libcurl (HTTP), libmicrohttpd (HTTP server features),
+  libnotify, libgcrypt, Python (plugins).
+
+The full Debian package list is in `Dockerfile.debian` — treat that as the
+authoritative dependency list.
+
+## Sequence
+
+```sh
+./autogen.sh        # generate configure from configure.ac
+./configure [opts]
+make -jN
+```
+
+`autogen.sh` runs `aclocal`, `autoheader`, `automake`, `autoconf`, `libtoolize`.
+
+## Useful `./configure` flags
+
+| Flag | Effect |
+|---|---|
+| `--enable-debug` | Debug build (the `configure-debug` script wraps this). |
+| `--disable-omemo` / `--disable-otr` / `--disable-pgp` | Drop encryption stack. |
+| `--disable-python-plugins` | C-only plugins. |
+| `--disable-notifications` | No libnotify. |
+| `--prefix=<path>` | Install prefix. |
+
+`./configure --help` prints the full set; flag names track upstream Profanity
+closely.
+
+## `configure-debug`
+
+Convenience script at repo root. Roughly:
+
+```sh
+./autogen.sh
+./configure --enable-debug --prefix=$(pwd)/install
+```
+
+Tweak in-script if you want different debug knobs.
+
+## `make` targets
+
+| Target | What |
+|---|---|
+| `make` | Builds the `profanity` binary (and `cproof` symlink, if rename has progressed to it). |
+| `make check` | Builds and runs the unit-test suite. |
+| `make clean` | Wipes object files. |
+| `make distclean` | Wipes generated configure artefacts too. |
+
+## Why this is host-unfriendly
+
+- Library version drift between the host (a workstation) and the canonical
+  Debian build means the host build can succeed while CI fails (or vice
+  versa) — bugs that don't reproduce in either env are very expensive.
+- `Dockerfile.debian` pins the OS and package set used by CI. Mirror it
+  locally instead of fighting the host.