feat(cli): add dotenv support, env var template, and bitcoind setup docs

[Olorunshogo]

Description

Notes to the reviewers

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

PR summary

Summary

• Add dotenvy and load .env at startup, before env_logger::init(), so every #[arg(env = "...")] value in src/ can be set via a file instead of shell exports. A missing .env stays silent; a malformed one now logs a warn! instead of failing silently.
• Add .env.example documenting every environment variable the CLI reads, grouped by command (global options, descriptors, backends, proxy, key management, transaction building, PSBT ops, compile, Payjoin, logging).
• Fix .gitignore: the entry was .env/, a directory-only pattern that never matched the .env file itself. Changed to .env.
• README.md:
  • Document the bitcoind/bitcoin-cli prerequisite for the rpc feature and the Justfile workflow, with an install snippet and a checksum-verification note.
  • Wire cp .env.example .env into both the install instructions and the regtest walkthrough.
  • Fix the $(just address) vs ${just address} footgun with an explicit note.
  • Renumber the regtest walkthrough steps to stay sequential after the new step was inserted.

Files changed

File	Change
Cargo.toml / Carg5"
src/main.rs	load .env on startup, warn on parse failure
.env.example	new
.gitignore	fix .env/ → .env
README.md	bitcoi step fixes

Test plan

                                                                      - [ ] `cargo build` an` succeed

• cp .env.example .env, set NETWORK, confirm it applies without a CLI flag
• Put an invalid line in .env, confirm a warn log appears and the app still starts
• git check-ignore .env reports it as ignored
• Walk the regtestend