Add --start-point flag for custom branch base

[opencode-agent]

I've implemented the --start-point flag to allow basing new branches on something other than the default (HEAD for local branches/issues, FETCH_HEAD for PRs).

Changes made to cmd/add.go:

1. Added new flag: --start-point - specifies the starting point for the new branch (e.g., branch, tag, commit)
2. Updated createFromPR, createFromIssue, and createFromLocal to use the flag when provided
3. Added usage examples

Usage:

# Create worktree from a specific branch
gh wt add my-feature-branch --start-point develop

# Create worktree from a specific commit
gh wt add my-feature-branch --start-point abc123

Closes #29

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[ffalor]

/oc use --base and -b for the flag. For PR's the base will be from the PR so this flag will need to be ignored.

[opencode-agent]

Done. Changes made:

1. Changed flag from --start-point to --base with -b shorthand
2. Updated examples to use --base
3. In createFromPR, now always uses FETCH_HEAD (ignores the --base flag since base comes from PR)
4. createFromIssue and createFromLocal still use the flag when provided

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