build: add Local.xcconfig override for per-developer signing

[datlechin]

Closes #1033.

Summary

Move per-developer signing values out of project.pbxproj and into a gitignored Local.xcconfig. Removes the git update-index --skip-worktree TablePro.xcodeproj/project.pbxproj workaround documented after #1028. Standard Apple xcconfig pattern (project-level base configuration + optional include of a developer-local file).

What this PR delivers (committed files only)

• Configurations/Shared.xcconfig: project-level defaults for DEVELOPMENT_TEAM, PRODUCT_BUNDLE_IDENTIFIER, CODE_SIGN_ENTITLEMENTS. Final line is #include? "../Local.xcconfig" (Apple's documented optional-include syntax).
• Local.xcconfig.example: copy-and-edit template, three commented overrides.
• .gitignore: ignores Local.xcconfig.
• CONTRIBUTING.md: rewrites the personal-team section to a cp + edit + build flow. No more pbxproj skip-worktree hack.

What you (maintainer) need to do in Xcode before merging

The override only takes effect after the project file is rewired. xcconfig values cannot override target-level Build Settings; the existing target-level entries must be removed so they fall back to the xcconfig.

1. Wire Configurations/Shared.xcconfig at the project level:

   • Project Editor (top of file list) → select the TablePro project (not a target) → Info tab → Configurations section.
   • For both Debug and Release, set "Based on Configuration File" → Configurations/Shared.

2. Remove redundant target-level overrides (so the xcconfig flows through):

   • TablePro app target → Build Settings → for both Debug and Release configs, delete:
     • DEVELOPMENT_TEAM (currently D7HJ5TFYCU)
     • PRODUCT_BUNDLE_IDENTIFIER (currently com.TablePro)
     • CODE_SIGN_ENTITLEMENTS (currently TablePro/TablePro.entitlements)
   • mcp-server target → Build Settings → delete DEVELOPMENT_TEAM (both configs).
   • Do not touch plugin targets (MySQLDriver, PostgreSQLDriver, etc.). They have unique bundle IDs (com.TablePro.MySQLDriver etc.) that must stay per-target. Their DEVELOPMENT_TEAM = "" already inherits from the project.

3. Verify:

   • Build TablePro Debug under the official D7HJ5TFYCU team. Should produce a binary signed exactly as before (no Local.xcconfig present, defaults from Shared.xcconfig apply).
   • codesign -d --entitlements - build/Debug/TablePro.app should still show D7HJ5TFYCU.com.TablePro.shared as the keychain access group.

The pbxproj diff from these changes is small (~16 deleted lines, ~4 baseConfigurationReference additions). Worth a careful diff review before committing.

Why this is the canonical Apple pattern

• #include? (with ?) is documented Xcode syntax for optional includes. Used in Apple's own sample code (Backyard Birds, Food Truck).
• Project-level base configuration with target-level pbxproj overrides removed is exactly how Apple structures sample projects when per-developer overrides are needed.
• No comments-as-instructions, no --skip-worktree, no shadow files. Just the standard build settings inheritance chain: SDK defaults → project xcconfig (Shared) → optional Local.xcconfig override → target build settings (left untouched for plugins).

Caveats

• Free personal teams still hit Apple's 10-bundle-IDs-per-7-days limit because the project signs around a dozen plugin bundles. Documented in CONTRIBUTING.md. xcconfig can't fix that.
• Existing contributors who ran git update-index --skip-worktree TablePro.xcodeproj/project.pbxproj should undo with git update-index --no-skip-worktree TablePro.xcodeproj/project.pbxproj before pulling this branch.

CHANGELOG

Not updated: this is a build/contributor-tooling change, not a runtime feature. The previous PR #1028 (which this supersedes for the personal-team flow) is still in [Unreleased]. Happy to add a Changed entry if you want one.

Test plan

• swiftlint lint --strict: no Swift changes.
• Banned-word check on diff: clean.
• After Xcode wiring: Release build under official team produces identical signing (codesign -d --entitlements - matches main).
• After Xcode wiring: contributor flow tested with cp Local.xcconfig.example Local.xcconfig → set DEVELOPMENT_TEAM → build succeeds.

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