Add Metabase exploration recipes cookbook

[andreagrandi]

Summary

Closes #16. Adds docs/RECIPES.md — a cookbook of six worked
end-to-end recipes for common Metabase exploration tasks, addressing
the need for clearer recipes for users and agents.

Each recipe states a goal and walks a command sequence where every
step feeds an ID or name into the next, with trimmed sample output and
a concise agent note where relevant:

1. Discover a database schema — database list → schemas →
   schema → table metadata → table fks → field summary/values
2. Inspect a saved question (card) — search → card get →
   card get --full → card run
3. Run a parameterized saved question — card params →
   card run --param, including the actionable invalid-parameter error
4. Analyze a dashboard — dashboard get → analyze → cards
5. Run a parameterized dashboard card — dashboard params list/values/search → run-card --param
6. PII-safe ad-hoc querying — query filter/query sql with
   redaction, what it misses, and blocked exports

Changes

• docs/RECIPES.md — new cookbook (six recipes)
• README.md — new ## Recipes section linking to the cookbook
• CHANGELOG.md — entry under [Unreleased]

Verification

• Every command was run against a live Metabase instance to confirm
  output format and behavior. Examples use generic e-commerce-style
  data but reproduce the actual output shapes mb-cli v0.2.0 produces
  (column headers, JSON keys, footer hints, redaction markers).
• make test passes. Docs-only change — no Go code touched.

Acceptance criteria

• Docs include practical Metabase workflows
• Examples match current CLI behavior
• Agent-specific guidance remains concise and actionable