Support configurable default --from / default parent branch

[dezren39]

Problem

Without --from, ez create always stacks the new branch on top of the current branch. Many workflows use a non-trunk integration branch (e.g. dev, staging, release/next) as the default base for new work. There is no way to configure a default parent.

Current behavior

• ez create foo stacks on whatever branch is checked out
• ez create foo --from main works but must be specified every time
• No config for "default base branch" distinct from trunk

Proposed solution

1. Add default_from (or default_parent) to .git/ez/stack.json
2. When --from is not specified and the current branch is trunk, use default_from if set
3. ez config set default-from <branch> to configure
4. Falls back to current behavior (stack on current branch) when unset

Use cases

• Teams that develop against dev and only merge dev → main for releases
• Repos with multiple long-lived integration branches
• Avoiding accidental deep stacking when user forgets --from

[dezren39]

relates to #2 but doesn't fully solve

[dezren39]

Implementation Update

Implemented in PR-A (feat/config):

• Added from config key (renamed from default_from for brevity)
• ez config set from <branch> to configure default parent
• When --from is not specified and the current branch is trunk, ez create uses the from config value
• Falls back to current behavior (stack on current branch) when unset

PR: rohoswagger#9