Document dependencies and check them at init time (#18)

[jepegit]

Summary

Closes #18.

issue-flow's scaffolded slash commands shell out to git and gh, but that was only discoverable by running a command and hitting a confusing failure. This PR makes the dependency list explicit both in docs and at install time.

• New src/issue_flow/dependencies.py declares REQUIRED_DEPENDENCIES (git, gh) with per-OS install hints and a docs link each. check_dependencies() uses shutil.which only. prompt_or_skip() prints a rich-formatted "missing deps" report and either:
  • bypasses the prompt when --skip-dep-check is set, or
  • bypasses the prompt when stdin is non-interactive (CI), or
  • calls typer.confirm and returns the user's decision.
• run_init / run_update now run the gate before any scaffolding; a decline raises typer.Exit(1). Both gained a skip_dep_check kwarg.
• CLI — issue-flow init and issue-flow update both expose --skip-dep-check for automation.
• README — new Prerequisites section above Installation covering git, gh (with an OS install table plus a gh auth login reminder), and uv, plus notes on the new check. Option tables and the Usage snippet updated to list the flag. uv is intentionally only in the README — it is an install-time prerequisite, not something the scaffolded workflow shells out to at runtime.
• HISTORY.md — new [Unreleased] bullet.
• Tests — new tests/conftest.py autouse fixture stubs check_dependencies to [] by default so unrelated tests stay deterministic on machines without git/gh. New tests/test_dependencies.py (11 cases) plus four new tests/test_init.py cases for all-present, --skip-dep-check, non-TTY bypass, and the decline path (typer.Exit(1), no scaffold written).

Test plan

• uv run pytest — 64 passed
• uv run ruff check src/ tests/ — clean
• uv run issue-flow init --help shows the new --skip-dep-check flag
• Manual: with shutil.which('gh') patched to return None, run_init(..., skip_dep_check=True) scaffolds successfully (20 files). Without the flag the full install-hint report is printed and the prompt is reached.

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