feat: Add Antigravity CLI and IDE local conversation provider (#8)

[tolmachevmaxim]

Summary

Closes #8

This PR adds full support for Antigravity CLI and IDE (antigravity) as a first-class local conversation source in Retain, along with key stability bug fixes, cross-provider global search, and session persistence improvements:

1. Antigravity CLI & IDE Support

• Two-Pass Merging: Discovers and merges transcript.jsonl (complete step history) and transcript_full.jsonl (untruncated rolling window of message content and tool outputs) by step_index to guarantee 100% complete conversations without missing initial prompts.
• Tolerant Parsing: Handles multiline JSON output, sanitizes unescaped control characters inside string literals, and parses Antigravity's ISO8601 UTC timestamps (including non-fractional second formats like YYYY-MM-DDTHH:MM:SSZ).
• Deterministic Identity & Deduplication: Generates deterministic message UUIDs and external IDs based on \(conversationId):\(stepIndex), preventing RUNNING -> DONE step duplicate orphans in SQLite.
• Noise Filtering & Content Cleaning: Strips Antigravity UI XML wrappers (<USER_REQUEST>, <ADDITIONAL_METADATA>, <USER_SETTINGS_CHANGE>, <SYSTEM_MESSAGE>) and filters internal agent system types (CONVERSATION_HISTORY, GENERIC, EPHEMERAL_MESSAGE, DIRECTORY_RULES).
• Metadata & Workspace Enrichment: Reads cache/conversation_metadata.json and cache/last_conversations.json to resolve conversation preview titles and decode file:// workspace paths.
• Live Sync & File Watching: Integrated with SyncService (full and incremental sync) and FileWatcher (monitoring candidate brain/ directories for .jsonl changes).
• Settings & UI Support: Integrated into ProviderRegistry, AppState, ContentView sidebar & empty states, SettingsView toggle & path diagnostics, and OnboardingView tool detector.
• Unit Tests: Full test suite in AntigravityParserTests.swift covering merge logic, multiline parsing, date handling, identity deduplication, content cleaning, and workspace decoding.

2. Analytics Stability & Performance Fixes

• Eliminated 100% Crash on Navigation: Fixed fatal crash in AnalyticsDataBuilder.fetchHourlyActivity caused by row["timestamp"] throwing GRDB decoding errors on non-standard timestamp strings.
• Fast SQL Aggregation: Converted hourly activity into direct SQLite strftime('%H', m.timestamp) aggregation, reducing query time to < 1 ms without loading hundreds of thousands of rows into memory.
• Defensive Date Decoding & Merging: Replaced crashing Dictionary(uniqueKeysWithValues:) with safe uniquingKeysWith: merging in buildContributionDays and fetchConsistencyTrend.
• Chart Domain Safety: Made providerDomain in TaskAffinityView and CorrectionRateView include all present providers dynamically, preventing Swift Charts domain mismatch precondition aborts.

3. Cross-Provider All-Time Search ("All Conversations")

• Added SmartFolder.all = "All Conversations" at the top of the Smart Folders sidebar section.
• Selecting it displays all conversations across all providers for all time and enables global cross-provider search.

4. Web Accounts Session Persistence Fix

• Fixed an issue where Claude.ai and ChatGPT web sessions disconnected upon app restart.
• KeychainHelper now attempts Data Protection keychain first and gracefully falls back to standard generic password keychain when running without access group entitlements (errSecMissingEntitlement -34018), ensuring session cookies and API keys are reliably saved and restored.

5. Settings & AI Features Polish

• Polished settings labels and section headers to clearly distinguish local CLI tools (Claude Code, Antigravity) from cloud-based Google AI Studio extraction.

Supported Locations

Scans standard Antigravity storage roots:

• ~/.gemini/antigravity-cli/
• ~/.gemini/antigravity/
• ~/.gemini/antigravity-ide/
• ~/.antigravity/

Testing

• Verified against on-disk corpus of 72 live Antigravity conversations (2,905 messages).
• Verified Analytics calculations and rendering on real database with 232,000+ messages.
• Verified Keychain roundtrip persistence across restarts.
• All unit tests and parser suites pass.