feat: support commit hash in --base flag

[amaya382]

Summary

• Allow bt add -b --base to accept commit hashes (full or short SHA) in addition to branch names
• Add IsCommitHash method to git executor using git rev-parse --verify
• When --base value doesn't match any local/remote branch, fall back to commit hash validation before erroring

Changes

• internal/git/branch.go: Add IsCommitHash() method
• cmd/bt/add.go: Add commit hash fallback in --base resolution, update help text
• README.md: Add --base with commit example, update command reference
• e2e/journey_remote_test.go: Add TestAddNewBranchWithCommitBase (full hash, short hash, invalid hash)
• e2e/README.md: Document new test case

Test plan

• task build passes
• task test passes (unit + E2E)
• golangci-lint run ./... passes
• Manual test: bt add -b feat/test --base <commit-hash> creates worktree correctly
• Manual test: bt add -b feat/test --base <invalid-hash> shows error

[Copilot]

Pull request overview

This PR extends bt add -b --base to accept commit hashes (full or short SHA) in addition to branch names, updating the CLI help/docs and adding E2E coverage for the new behavior.

Changes:

• Add git executor support for validating whether a ref resolves to a commit.
• Update bt add base resolution to fall back to commit-hash validation when a branch isn’t found.
• Add E2E coverage + documentation updates for --base <commit> usage.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
internal/git/branch.go	Adds a helper to validate that an input resolves to a commit object via git rev-parse.
cmd/bt/add.go	Adds commit-hash fallback for --base and updates CLI help text.
e2e/journey_remote_test.go	Adds E2E coverage for full hash, short hash, and invalid hash base cases.
e2e/README.md	Documents the new E2E test case.
README.md	Updates the command reference to mention --base supports branch/commit.

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[Copilot]

IsCommitHash is named like it validates a hex SHA, but git rev-parse --verify <ref>^{commit} will also succeed for non-hash commit-ish values (e.g. HEAD, tags, or other refs). This makes the API misleading and can unintentionally broaden what --base accepts. Consider either renaming this to something like IsCommitRef/IsCommitish, or tightening the check (e.g., first require the input to match a hex SHA pattern of an acceptable length) so the behavior matches the name/help text.

Suggested change

	// IsCommitHash checks if the given string resolves to a valid commit object
	func (e *Executor) IsCommitHash(ref string) bool {
	// IsCommitHash checks if the given string looks like a hex SHA and resolves to a valid commit object
	func (e *Executor) IsCommitHash(ref string) bool {
	ref = strings.TrimSpace(ref)
	// Require a plausible abbreviated or full SHA-1 (Git allows abbreviations, typically >= 4 chars)
	if len(ref) < 4 || len(ref) > 40 {
	return false
	}
	for _, c := range ref {
	if !((c >= '0' && c <= '9') ||
	(c >= 'a' && c <= 'f') ||
	(c >= 'A' && c <= 'F')) {
	return false
	}
	}

[Copilot]

Now that --base supports commit hashes, this error message is misleading for invalid hashes because it only mentions branches/remotes. Consider updating it to indicate the value wasn’t found as either a branch or a commit, so users understand why a hash-like value failed.

Suggested change

	return fmt.Errorf("base branch '%s' not found locally or on any remote", addBaseBranch)
	return fmt.Errorf("base '%s' not found as a local/remote branch or commit", addBaseBranch)

[Copilot]

This command reference row previously documented --no-fetch but now omits it. If --no-fetch is still supported, consider keeping it in the short description while adding the new --base mention to avoid a docs regression.

Suggested change

	| `bt add <branch>` | Add worktree (`-b` for new branch, `--base` for base branch/commit, auto-fetches remotes) |
	| `bt add <branch>` | Add worktree (`-b` for new branch, `--base` for base branch/commit, auto-fetches remotes unless \`--no-fetch\`) |