Emit primary-key markers and join keys in OKF export

[Obolrus]

Why

The OKF exporter emits relationships without concrete join keys and never marks a mart's primary key, even though it already reads isPrimaryKey from the OWOX schema and emits FK to [Target] notes. As a result any OKF consumer that renders the model — e.g. the Model Canvas ERD — draws every relationship as a bare line with no join columns. The picture of the model is incomplete: you can see that two marts are related, but not on what.

Measured on the current example bundle, parsed by the Model Canvas OKF parser:

NODES: withPK  = 0
EDGES: withKeys = 0   → every relationship is a keyless line

What this PR changes

1. Exporter (tools/odm-to-okf-export/export.py)

• Primary-key markers. render_schema_section now prefixes the PK column's description with PK. (cell order: PK. → description → FK to […], matching what the canvas serializer produces). The data was already available via isPrimaryKey; it just wasn't written out.
• Join keys in ## Joins. _render_joins_section now emits - [Target](./target.md) — left = right`` when the key is derivable — i.e. this mart has a column named the same as the target mart's primary key. OWOX's blendedFieldsConfig carries no explicit join columns, so a link with no matching column is left bare (unchanged behaviour for those).
• --folder flag. Optional override for the bundle subfolder name (defaults to the slugified project title, as before).

No cardinality is emitted — OWOX Data Marts has no such concept, and the canvas treats its absence gracefully.

2. Example bundle (bundles/…)

Regenerated with the fix so it renders join keys on the canvas, and cleaned up:

• Scoped to a single storage (BigQuery [Common]) so it is a coherent e-commerce model — 24 marts. The previous 27-mart example silently mixed marts from several storages (some same-named ad marts actually lived in Snowflake / other storages); those 3 out-of-storage marts are dropped and a few ad marts now resolve to their BigQuery counterparts.
• Renamed demo-project → e-commerce, so adding further example models later doesn't leave a generic demo-project sitting next to them.

After the fix, the same parser reports:

NODES: withPK  = 18 / 24
EDGES: withKeys = 9 / 20   (remaining keyless targets have no PK — connector/aggregate marts)

Keyed relationships now include e.g. orders → sessions: session_id = session_id, orders → customers: customer_id = customer_id, purchases → products: product_id = product_id, sessions → visitors: visitor_id = visitor_id.

How to verify

Import https://model.owox.com/?okf=https://github.com/OWOX/models/tree/<this-branch>/bundles/e-commerce and confirm the ERD shows join-key fields on the related marts instead of bare lines.