feat: make git remote name configurable instead of hard-coding 'origin'

[bhagyamudgal]

Context

Raised in PR #1 review (comment). Currently the tool hard-codes origin as the remote name in several places:

• gitFetch() in src/lib/git.ts — git fetch origin --prune
• create command in src/commands/create.ts — origin/${opts.name} branch lookups and origin/${base} fallback

This breaks workflows where the primary remote is named something other than origin (e.g., upstream, main).

Proposed Solution

Add an optional REMOTE key to .worktreerc config (defaulting to origin):

DEFAULT_BASE=origin/dev
REMOTE=origin

Then use the configured remote name throughout:

• gitFetch(remote) instead of gitFetch() with hard-coded origin
• Branch lookups use ${remote}/${name} instead of origin/${name}

Files to Change

• src/lib/config.ts — add REMOTE to config schema with default "origin"
• src/lib/git.ts — gitFetch accepts a remote parameter
• src/commands/create.ts — use config.REMOTE instead of "origin" string literals

Notes

Deferred from PR #1 as it requires coordinated changes across config, git wrappers, and the create command. Not a bug — origin is the standard convention — but a useful enhancement for non-standard setups.