Add --summary flag for compact agent-friendly nutrition snapshot

[ali5ter]

Problem

The --report output is comprehensive but ~70+ lines for a 7-day window. Several use cases benefit from a denser format:

1. Quick CLI glance — "how was this week?" in ~15 lines.
2. AI assistant ingestion — feeding nutrition data to a health advisor, coach, or LLM. The long-form report consumes context tokens; a structured summary is cheaper and more parseable.
3. Daily standup / journaling — a one-paragraph copy-paste.

Proposal

Add a --summary flag that emits a fixed-shape, ~15-line block. Example output (assumes [targets] configured per the targets-config issue):

wwlog summary  ·  2026-04-26 → 2026-05-02  ·  7 days

Calories      1713 kcal/day    target 1700–1900    ✓ in band (6/7 days)
Protein        160 g/day       target 130–150      ✓ above floor (7/7)
Carbs          148 g/day       target 158–228      ⚠ below floor (4/7)
Fat             58 g/day       target  31–54       ⚠ above ceiling (5/7)
Fiber           28 g/day       target ≥26          ✓ on target (5/7)
Sodium       1840 mg/day       target ≤2300        ✓ under ceiling (7/7)
Added sugar     18 g/day       target ≤35          ✓ under ceiling (7/7)
Points          20 / 23 avg    0 days over budget  ✓
Skipped meals    3   (lunch ×1, dinner ×2)
Eating window  09:24 → 19:48 avg  (target last meal ≤20:00)  ✓
Alcohol         24 fl oz beer (1 day, 11pt)

Format goals:

• Fixed-width columns
• One line per metric
• Hit/miss marker per line
• "Days on target" count where relevant
• Plain ASCII for terminal-or-agent ingestion (no ANSI escapes by default; honour --color if added later)

Acceptance criteria

• New --summary flag, mutually exclusive with --json and --report
• Uses configured targets when present; falls back gracefully if not
• Available as --export summary to file (extension .txt)
• Documented in README under "Usage" and "Options"
• Includes skipped-meal count and eating-window range when computable

Notes

• Depends on the targets-config issue to deliver the hit/miss column.
• Future variant: --for advisor could emit the same data with a short header explaining context, optimised for LLM ingestion.
• "Skipped meals" should respect a configurable threshold (e.g., a meal with zero entries logged).

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🤖 Generated with over-50s-health-advisor (Claude Code agent) on behalf of Alister