docs: slim the agent conventions

The founding rule restated SPEC.md, which stays the source of truth
for the confinement budget. Commit bodies gain a length cap, and a
commit must build and work on its own; bisectability outranks the
one-component-per-commit split when the two collide.

AGENTS.md

@@ -1,18 +1,5 @@
 # AGENTS.md
 
-Conventions for working in this tree. They apply to every commit; the
-whole history follows them.
-
-## The founding rule
-
-The SPEC states it; this file makes it operational. The host process
-never calls `mount`, `umount2`, `unshare`, `setns`, `pivot_root`, or
-any of the new mount API. New code stays inside the confinement budget
-(SPEC, "Confinement budget"). Test T6 in `vm.rs` guards the rule; keep
-it green. `unsafe` code lives in `krun.rs` (the FFI) and in explicit
-`libc` call sites; every unsafe block carries a comment stating the
-invariant that makes it sound.
-
 ## Before every commit
 
 Run, in order, and make sure they are clean before committing:
@@ -60,8 +47,12 @@ Rules:
   first letter, no trailing period, max ~72 characters.
 - Body (optional): separated by a blank line, wrapped at 72 columns;
   explain why rather than what. Reference issues as `#123`.
+  Keep body short for small changes (max. 4 lines) and expand into
+  max. 3 paragraphs for big changes.
 - A commit touching several components should be split into one commit
   per component when practical; otherwise use the dominant scope.
+  **Never break bisectability.**
+  Each commit should build and work on their own.
 - **No merge commits.** Integrate work by rebasing onto the target
   branch; the history stays linear.