feat: add semantic selectors and session management

[tosh7]

Summary

• Semantic selectors: --text and --type/--index options for tap and fill commands, enabling label-based and type-based element targeting without coordinates or ref numbers
• Session management: era session connect/list/disconnect with persistent device sessions (~/.era/sessions.json), --session option on all device commands, and cached scale factor auto-detection

Changes

Phase 3a: Semantic Selectors

• snapshot.rs: find_by_text() (case-insensitive partial match, prefers enabled elements), find_by_type_index() (type + 0-based index)
• cli/mod.rs: resolve_live_element() — live UI tree search via idb describe-all
• 4-way exclusive targeting: --ref / --text / --type / -x -y (enforced by clap constraints)
• 21 tests added

Phase 3b: Session Management

• New session.rs (447 lines): SessionStore with file-backed CRUD, resolve_device() priority chain (--session > --device > default session)
• --session added to: Snapshot, Tap, Fill, TapRegion, Swipe
• --device changed to Optional (falls back to session or default)
• resolve_scale() supports session cached scale
• 13 tests added

Housekeeping

• Removed firebase-debug.log from tracking, added to .gitignore

Usage Examples

# Semantic selectors
era tap -d <device> --text "カートに入れる"
era tap -d <device> --type Button --index 2
era fill -d <device> --target "メールアドレス" "test@example.com"

# Session management
era session connect --name main -d "iPhone 16 Pro"
era tap --session main --text "ログイン"
era tap --text "ログイン"  # uses default session
era session list
era session disconnect --name main

Test plan

• All 193 tests pass (90 lib + 18 coordinate + 56 regression + 12 retry + 17 scale)
• Semantic selector 4-way exclusivity verified via clap constraint tests
• Session file I/O round-trip tests with parallel-safe temp directories
• Merge conflict resolution verified (handle_tap/handle_fill parameter integration)