feat: PR description draft from config and forge templates

[simtel12]

Summary

This change seeds the PR/MR description editor with smarter defaults instead of a single hard-coded outline.

Precedence (first match wins):

1. [repo].pr_description_template in .pilegit.toml — path (relative to repo root) to a markdown file. If the file exists and is non-empty, its contents seed the editor. The placeholder {{subject}} is replaced with the commit title when present in the template.
2. Forge-specific templates — GitHub looks for the usual PR template locations (e.g. .github/pull_request_template.md and the other documented single-file paths). GitLab reads .gitlab/merge_request_templates/*.md, preferring Default.md when present, then other files in sorted order.
3. Built-in fallback — a short outline with Description and Test Plan sections and the commit title under Description.

The submit flow in the TUI now calls forge::pr_description::compose_initial_draft with the current forge, repo config, and subject. Config loading for the TUI entry path uses Config::load_or_default so default [repo] fields (including the new option) are applied consistently.

Forge trait — pr_description_draft_hint is the extension point for per-forge discovery; only GitHub and GitLab implement it today; other forges use config + built-in.

Test plan

• cargo test — includes unit tests in src/forge/pr_description.rs (GitHub template path, config overrides forge, built-in fallback).
• Config template: add pr_description_template = "path/to.md" under [repo] in .pilegit.toml, put {{subject}} in the file, run submit for a patch and confirm the temp editor file contains the substituted title.
• GitHub: in a repo with .github/pull_request_template.md, submit (or the code path that opens the description editor) and confirm the template body appears when pr_description_template is unset.
• GitLab: with .gitlab/merge_request_templates/Default.md (or another .md in that directory), confirm the same when using GitLab forge type.
• No templates: empty/minimal repo with no config template — confirm the built-in Description / Test Plan draft still appears with the commit subject.

[hokwangchoi]

Please fix clippy. Thank you!

[gregorios-leach-ck]

| Please fix clippy.

Fixed!