feat: markdown by default, --json for structured

[DTTerastar]

Summary

Default output is now narrow, Fitdown-style markdown — date-grouped headings, one bullet per non-zero field, no wide tables. Pass --json (or --format json) for the full structured row.

This is a breaking change to the CLI's contract: any script or agent that pipes crono-export X | jq … will need to add --json. The reasoning: agents handling the output inline read narrow markdown better than raw JSON, and human users running it ad-hoc in a terminal get a readable result instead of a wall of JSON.

Layout per endpoint

• servings — ## per date, ### per food (group · name · qty), bullets for non-zero nutrients. Uses reflection over the typed struct and splits CamelCase suffixes (EnergyKcal → Energy (kcal), B12Mg → B12 (mg)).
• nutrition — ## per date, alphabetical bullets for non-zero columns.
• biometrics — ## per date, bullet per metric (Metric: amount unit).
• exercises — ## per date, single line per session (Running, 30 min, 320 kcal (Cardio)).
• notes — ## per date with note body; falls back to dumping all non-empty fields if a Note column isn't present.

Markdown suppresses zero-valued nutrients to stay scannable; the trailer points users at --json for the full row.

Touchpoints

• New cmd/format.go with AddFormatFlags, emit(cmd, kind, v), and per-kind renderers.
• Every subcommand registers the new flags and calls emit(...) in place of the old emitJSON(...).
• prime, root --help, and the README updated to describe the new contract.

Live samples

crono-export nutrition --today (default markdown):

## 2026-04-25
- B12 (Cobalamin) (µg): 3.27
- Calcium (mg): 177.83
- Carbs (g): 75.72
- Cholesterol (mg): 569.46
- Energy (kcal): 1576.25
- Fat (g): 100.92
- Fiber (g): 6.38
- Iron (mg): 6.09
- Protein (g): 99.42
…

crono-export biometrics --days 30:

## 2026-04-10
- Weight: 237 lbs

Test plan

• go build ./... clean
• go vet ./... clean
• nutrition --today renders markdown with zero-suppressed bullets
• servings --today renders per-date / per-food sections with non-zero nutrients
• biometrics --days 30 renders per-date bullets with metric/unit
• nutrition --today --json and --format json both still emit full pretty-printed JSON
• nutrition --today --format bogus exits non-zero with a clear message
• --help on every subcommand lists --format and --json
• crono-export prime updated to describe both formats
• Eyeball: have an agent call commands without --json and see if the output is more useful in chat

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