Add `eko search` command to search file contents across snapshots

[kavix]

🎯 Goal

Implement an eko search <pattern> command that performs a text search across all files in a given snapshot (or all snapshots) and reports matching file paths and line numbers, similar to grep -r but scoped to Eko snapshots.

💡 Expected Usage

# Search for a string in the latest snapshot
eko search "TODO"

# Search in a specific snapshot
eko search "TODO" --snapshot 8c9d1a2f

# Search across all snapshots
eko search "deprecated" --all

🛠️ Implementation Steps

1. Create cmd/search.go with Cobra command searchCmd.
2. Walk .eko/snapshots/<id>/ file tree and use bufio.Scanner to search each text file for the pattern (skip binaries using heuristics).
3. Print matching file path, line number, and surrounding context (like grep -n).
4. Support --all flag to iterate over all snapshot IDs from the DB.
5. Add unit tests in cmd/commands_test.go.

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📖 See full details in GOOD_FIRST_ISSUES.md — Issue #16
🌟 Great for first-time contributors! Read CONTRIBUTING.md to get started.