Move per-contributor team override from pbxproj to a gitignored xcconfig

[datlechin]

Follow-up to #1028.

#1028 unblocked external-contributor builds, but the personal-team setup still requires editing TablePro.xcodeproj/project.pbxproj in Xcode (Team, Bundle Identifier, Code Signing Entitlements) and then telling git to ignore those edits via git update-index --skip-worktree.

That works but is fragile:

• --skip-worktree is per-clone, easy to forget on a fresh checkout.
• Any future Xcode UI tweak by a contributor risks accidentally committing their team prefix back.
• It's not the Apple-recommended pattern.

Proposal

Move the three overridable values into a gitignored xcconfig file. The repo already uses Secrets.xcconfig (CONTRIBUTING.md:12) for similar purposes, so the infrastructure is in place.

Roughly:

1. Add Local.xcconfig to .gitignore (with a committed Local.xcconfig.example).
2. In TablePro.xcodeproj, set the Debug build settings for DEVELOPMENT_TEAM, PRODUCT_BUNDLE_IDENTIFIER, and CODE_SIGN_ENTITLEMENTS to read from the xcconfig (\$(inherited) chain).
3. Default values stay set to the official team / com.TablePro / TablePro/TablePro.entitlements so unmodified clones keep working.
4. Replace the personal-team section in CONTRIBUTING.md with a 3-line block: copy the example, set DEVELOPMENT_TEAM, set PRODUCT_BUNDLE_IDENTIFIER, optionally set CODE_SIGN_ENTITLEMENTS=TablePro/TablePro.Debug.entitlements.

Outcome

• No more pbxproj edits for personal-team contributors.
• git status stays clean by construction; no --skip-worktree needed.
• Official Release builds are unchanged (xcconfig is empty / absent → defaults apply).