feat: improve dev shellenv and link PR branches to origin

[brosu]

Summary

• Add just dev-shellenv helper for running from source with shell completion in any directory
• Generate a local go.work (ignored) to keep go run working outside the wt repo
• Link wt pr worktree branches to the PR’s origin/ when available
• Document dev-shellenv usage in Development section

Testing

• Not added (no existing test coverage for gh/remote PR flows)

[timvw]

Hey @brosu, thanks for this PR! The ideas here were solid — both the dev-shellenv recipe and the branch linking logic.

I only noticed this PR after I'd already merged #39, which refactored wt pr/wt mr to look up the actual branch name from the PR/MR (instead of using pr-N/mr-N). That made the linkGitHubPRBranch function here largely redundant, since the new flow already fetches and tracks the real branch.

The good news is that all the useful parts from this PR have now been covered across three follow-up PRs:

• feat: use actual branch name for pr/mr checkout #39 — Branch name lookup for wt pr/wt mr (replaces the Go code changes here)
• feat: ensure upstream tracking for pr/mr worktree branches #40 — Upstream tracking (--set-upstream-to) for pr/mr worktree branches (the one piece feat: use actual branch name for pr/mr checkout #39 didn't cover)
• chore: add dev-shellenv recipe for running wt from source #41 — just dev-shellenv recipe, go.work.sum in .gitignore, and expanded Development docs (the non-Go parts of this PR)

Closing this one since everything is covered. Thanks again for the contribution!