Add harvest command to gather dependabot-style bump messages

[charphi]

A new command that reads git log commit subjects, filters Dependabot-style "bump" messages, and adds them as entries in the Changed section of the [Unreleased] changelog block.

[charphi]

Implementation Approach:

Use git log TAG..HEAD --pretty=format:"%H %s" to retrieve Dependabot
commit messages in chronological order between a specified tag and HEAD.

Core steps:

1. Detect tag range - Automatically use latest tag, allow --from-tag override
2. Parse commits - Filter using regex (support scoped packages, pre-release versions):
   ^Bump (@?[\w/-]+) from ([\d.\w-]+) to ([\d.\w-]+)
3. Extract & group by package - For each package, use the first commit's "from"
   version and the last commit's "to" version to avoid fragile version arithmetic
4. Handle edge cases - Skip entries where version direction changes (upgrade→downgrade)
5. Deduplicate - Check existing changelog entries before inserting
6. Format entries - - Bump [package] from [first-from] to [last-to]
7. Insert into changelog - Append to [Unreleased] → Changed section

Command usage:

heylogs harvest                           # Uses latest tag
heylogs harvest --from-tag v1.5.0        # Specify explicit tag
heylogs harvest --from-tag v1.5.0 --grep "Bump|Update"  # Custom pattern

Edge cases to handle:

• Non-semantic versioning (warn or skip)
• Version direction changes (keep separate or skip)
• Rebased commits (use timestamps for ordering)
• Missing tags (fallback to date range or earliest commit)