docs: record the module-shape, cleanup, and review-shape conventions

[lneto]

Conventions the tree already follows but AGENTS.md did not record, each drawn from a place a review missed them.

Lua style

• A module that makes objects returns a class or namespace table and constructs through .new or a :__call method on the class (hkdf, inet), never a bare function or a metatable wrapped around the module; the underlying handle sits in a named field, not one prefixed with an underscore; the instance metatable is named for what it is, not mt.
• A proxy's metamethods do not allocate per access, as bpf.map's view shows.
• CAPS <const> marks a scalar; a lookup, dispatch, or allow-list table is a lowercase name by role (codecs, tokens), never caps.

Deciding what to change

• A function's contract — read-only, never-sleeps, what it returns — is read from its body, not inferred from its name or its place in a method table (connmark reads and writes despite sitting among read-only accessors).
• A call the conventions here already settle is decided and noted, not escalated; a question is reserved for a genuine fork no rule, precedent, or test resolves.

Tests

• The cleanup undoes everything any case can create, not only what the happy path stops inline, so a case that fails before its own teardown does not wedge the next run.

Reviewing

• A new module is read for its shape against its nearest sibling, not only its logic; correct code in a shape the tree does not use is a finding, and confirming the shape means grepping for the peer.
• The base can be the outdated one: when new code diverges from it, check which side is right before aligning, rather than pulling the new code down to match a sibling that is itself behind.

Docs only.

• A re-review re-fetches the author's branch and reads what changed there, not the review branch you built last round.
• Once it is clean, the verdict that a PR can merge is an Approve, not a comment saying it looks good.
• Being told to post is not a license to post words the maintainer has not read: show the exact text, get the go-ahead on it, then post.
• An artifact with no test of its own (example, script) is run directly before a verdict, not just built.
• Reshaping or renaming an API means updating every consumer, including ones in stacked pull requests.