chore: add presubmit workflow and pre-commit quality checks

[galz10]

Summary

Introduce a unified presubmit and pre-commit quality flow for IDX0, including local scripts, repo-managed git hooks, deterministic lint/docs configs, and a new multi-job GitHub Actions presubmit workflow.

Details

• Added scripts/presubmit.sh with lint|docs|test|fast subcommands.
• Added scripts/install-hooks.sh plus repo hook path support via .githooks/pre-commit.
• Added deterministic config files:
  • .swiftformat
  • .swiftlint.yml
  • .markdownlint.json
  • .lychee.toml
• Added .github/workflows/presubmit.yml with:
  • lint-docs (SwiftFormat, SwiftLint, markdown lint, link checks)
  • tests (setup + full xcodebuild tests)
  • maintainability (phase-1 report-only via continue-on-error: true)
• Updated contributor-facing docs and PR template to reflect the new quality flow.

Related Issues

N/A

How to Validate

1. Verify script shape and workflow syntax:
   • bash -n scripts/presubmit.sh scripts/install-hooks.sh .githooks/pre-commit
   • ruby -e 'require "yaml"; YAML.load_file(".github/workflows/presubmit.yml")'
2. Install hooks and verify hook path setup:
   • ./scripts/install-hooks.sh
   • Expected: prints core.hooksPath set to .githooks.
3. Verify presubmit help:
   • ./scripts/presubmit.sh --help
4. Run app tests:
   • xcodebuild -project idx0.xcodeproj -scheme idx0 -destination 'platform=macOS' test
   • Current result in this environment: fails on pre-existing SessionServiceTests assertions (testNiriColumnResizePersistsPreferredWidths, testNiriDefaultTileSizesApplyOnlyToNewTiles, testNiriItemResizePersistsPreferredHeights).
5. Optional full local presubmit after installing tools:
   • brew install swiftformat swiftlint lychee
   • npm install -g markdownlint-cli
   • ./scripts/presubmit.sh fast
   • ./scripts/presubmit.sh lint
   • ./scripts/presubmit.sh docs

Pre-Merge Checklist

• lint-docs check is green
• tests check is green
• maintainability result reviewed (phase 1 report-only)
• Added/updated tests (if needed)
• Updated relevant documentation and README (if needed)
• Noted breaking changes (if any)
• Tested on macOS