OKF v0.2 output structure

A converted bundle is an ordinary directory tree:

bundle/
  index.md              # root index: declares okf_version, lists concepts + subdirs
  overview.md           # a concept
  metrics/
    index.md            # per-directory nav (no frontmatter)
    revenue.md          # a concept

Key rules binder enforces on emit:

One concept per non-reserved .md. A concept’s ID is its bundle-relative path minus .md.
okf_version lives only in the root index.md (spec §12), and the root index is the only index carrying frontmatter (spec §8).
```
---
okf_version: "0.2"
---
```
Every concept has a non-empty type — the one hard-required field (spec §11). All other frontmatter keys are optional and preserved as-is, including keys binder does not understand (spec §4).
A generated stamp records the conversion, added only when the concept does not already carry one:
```
generated:
  at: "2023-11-14T22:13:20Z"
  by: binder/0.3.0
```
Reserved-name source files are never dropped. A source index.md/log.md is renamed to <stem>-note.md (with a numeric suffix on collision) so binder can generate its own index.md, and the rename is reported.
Links are rewritten to bundle-relative-absolute form. A body link [t](../a/b.md#sec) that resolves to a concept becomes [t](/a/b.md#sec).

For the full reference (including reserved files, the concept/edge vocabulary, and validation rules), see the user guide.