feat(git-extension): add configurable Conventional Commit support (#3390)

[dhruv-15-03]

Summary

Implements issue #3390: adds a configurable commit_style option (fixed | conventional) to the bundled git extension's auto-commit feature.

• commit_style: fixed (default) — unchanged, static configured messages.
• commit_style: conventional — the speckit.git.commit hook now instructs the agent to generate a Conventional Commit message from the diff and pass it explicitly to auto-commit.sh / auto-commit.ps1 as a second argument.
• If commit_style: conventional is set but no generated message is supplied, the scripts fail loudly (stderr message + exit 1) instead of silently committing with the fixed message — while still short-circuiting cleanly when there are no changes to commit.

Changes

• extensions/git/config-template.yml, extensions/git/git-config.yml: new commit_style option with docs.
• extensions/git/scripts/bash/auto-commit.sh: optional [generated_message] arg, commit_style parsing, conventional-mode enforcement (ordered after the "no changes" check).
• extensions/git/scripts/powershell/auto-commit.ps1: mirrored PowerShell implementation.
• extensions/git/commands/speckit.git.commit.md: documents the new "Commit Message Styles" section and updated execution/config/graceful-degradation guidance.
• extensions/git/README.md: documents the new option in the configuration example.
• tests/extensions/git/test_git_extension.py: new TestAutoCommitBashCommitStyle / TestAutoCommitPowerShellCommitStyle regression tests covering fixed-default, conventional success, conventional missing-message failure, and no-changes short-circuit.

Testing

• pytest tests/extensions/git/test_git_extension.py -v — all bash/PowerShell tests requiring bash/pwsh skip consistently in this environment (matching existing TestAutoCommitBash/TestAutoCommitPowerShell behavior via @requires_bash / HAS_PWSH).
• Manually validated all four scenarios (fixed default, conventional success, conventional missing-message failure, no-changes short-circuit) end-to-end against auto-commit.ps1 using Windows PowerShell, since pwsh (PowerShell 7) isn't installed in this environment.
• pytest tests/ (full suite): 3614 passed, 295 skipped, 39 failed — all 39 failures are pre-existing, environment-specific (Windows symlink-permission tests and pwsh-only tests), unrelated to this change.
• ruff check extensions/git tests/extensions/git: all checks passed.

Fixes #3390

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This PR was prepared with the assistance of GitHub Copilot (model: claude-sonnet-5, operating autonomously) on behalf of @dhruv-15-03.

[Copilot]

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[grafvonb]

/lgtm