Import Postman collections (v2.1) and environments #47

Why

Postman is where most teams' requests live today, and "can I bring my collection?" is the first question from anyone evaluating a replacement. apic import only reads OpenAPI 3 (internal/cli/import.go:17-22). Bruno's success with migration shows the value.

What

apic import collection.postman.json -o api --postman-env staging.postman_environment.json

Acceptance criteria:

• Detects a Postman v2.1 collection by its info.schema; v2.0 is accepted where the shapes coincide, v1 is refused with a message.
• Folders become files (<folder>.http), nested folders join with -; requests become named requests (kebab-case of the name, de-duplicated); the request description becomes # @description.
• URL, method, headers, query, path variables (:id → {{id}}), raw/JSON/urlencoded/form-data bodies, and {{var}} placeholders map 1:1. GraphQL bodies map onto the GraphQL form once that lands.
• Auth blocks map: bearer → # @auth bearer, basic → # @auth basic, apikey → # @auth apikey (once that exists, else a header), awsv4 → # @auth aws, oauth2 client credentials → # @auth oauth2. Others are listed in skipped.
• Collection and environment variables become http-client.env.json ($shared for collection-level, one environment per --postman-env file); values marked secret go to the private file.
• pm.test scripts: translate the trivial forms (pm.response.to.have.status(200), pm.expect(jsonData.x).to.eql(y), header equals) to # @assert; anything else is reported in skipped with the request and a one-line reason.
• --json output extends the existing {"files", "requests", "base_url", "env_file", "skipped"} shape.

Where

New internal/postman/ (decoder as a small hand-written model like internal/openapi/model.go; no library), internal/cli/import.go (dispatch on file type), reuse the file-writing helpers from the OpenAPI importer.

Docs

docs/cli.md (import section), README command table row, docs/cheatsheet.md, docs/cookbook.md ("Migrate a Postman collection"), docs/comparison.md (Postman import row).

Tests

A fixture collection under internal/postman/testdata/ covering folders, every body type, every auth type, path variables and three pm.test shapes; golden .http output; environment mapping; refusal of v1.