feat: `cora gain` — local productivity stats + viral sharing

[ajianaz]

Summary

Add cora gain subcommand that tracks local review statistics and enables viral sharing of productivity gains — all privacy-first (no telemetry, local SQLite).

Motivation

• User engagement: Shows tangible value (time saved, issues caught)
• Virality: --share generates screenshot-ready TUI output for Twitter/LinkedIn
• Retention: Users see ROI, less likely to churn
• Community: --badge generates markdown badge for README

Data Model

~/.cora/gain.db (SQLite)

CREATE TABLE reviews (
    id TEXT PRIMARY KEY,
    timestamp TEXT NOT NULL,
    mode TEXT NOT NULL,          -- staged, branch, scan, unpushed
    files_changed INTEGER,
    lines_changed INTEGER,
    issues_found INTEGER,
    issues_by_severity TEXT,     -- JSON: {"critical":1,"major":3}
    tokens_used INTEGER,
    duration_ms INTEGER,
    provider TEXT,
    model TEXT,
    project_hash TEXT,           -- hashed project path (privacy)
    has_rules_fired BOOLEAN,
    has_context_chain BOOLEAN
);

CREATE TABLE issues (
    id TEXT PRIMARY KEY,
    review_id TEXT REFERENCES reviews(id),
    severity TEXT,
    category TEXT,
    file_path TEXT,
    rule_id TEXT,
    was_fixed BOOLEAN
);

CLI Interface

# Dashboard (default)
cora gain
# TUI-style output:
# ┌─ Cora Gain ─────────────────────┐
# │ Reviews: 142   Issues caught: 89│
# │ Time saved: ~14h   Tokens: 2.1M │
# │ Top provider: openai/gpt-4o     │
# └─────────────────────────────────┘

# Period stats
cora gain --week
cora gain --month

# Share (copy-ready text for social)
cora gain --share

# Markdown badge for README
cora gain --badge

Tracking Hooks

Hook into review.rs pipeline:

1. Pre-review: record mode, files_changed, lines_changed, start timer
2. Post-review: record issues_found, tokens_used, duration_ms, provider, model
3. Per-issue: record severity, category, rule_id (from rules engine)

Privacy

• All data local at ~/.cora/gain.db
• Project paths hashed (not stored in plain)
• No network calls, no telemetry
• --share output includes NO file paths or code content

Implementation

1. Add Gain variant to Commands enum in main.rs
2. Create src/commands/gain.rs with dashboard, period, share, badge sub-commands
3. Add tracking hooks in src/commands/review.rs
4. Schema migration in src/gain/mod.rs
5. TUI output formatting
6. Tests for all modes

Acceptance Criteria

• cora gain shows dashboard with all-time stats
• cora gain --week/--month shows period stats
• cora gain --share generates social-ready text
• cora gain --badge generates markdown badge
• Review pipeline records stats to SQLite
• Zero network calls / telemetry
• Tests pass