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

[Olorunshogo]

PR Summary

Summary

• dotenvy: add dotenvy = "0.15" and load .env at startup before env_logger::init(). A missing .env stays silent; a malformed one logs a warn! instead of failing silently.
• .env.example: new file documenting every environment variable the CLI reads, grouped by command area (global options, descriptors, backends, proxy, key management, transaction building, PSBT ops, Payjoin, logging).
• .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 note pointing at the official verification guide. Wire cp .env.example .env into the install instructions and the regtest walkthrough. Fix the $(just address) footgun with an explicit note. Renumber the regtest steps sequentially after the new step was inserted.
• docs/summary.md: new capability reference doc covering all commands, backends, and setup paths.

Notes to the reviewers

• dotenvy::dotenv()'g .env stays silent(the common case), but a
  malformed one now lor::init()` runs, instead of failing silently.
• .env.example cover value in src/,including POLICY/TYPE for
  the compile commanble names match theexisting clap definitions
  as-is; renaming themeaking anyone alreadyusing them as shell exports.
• The Bitcoin Core insips checksumverification for brevity; added a
  note pointing at thetion guide instead ofinlining the full steps.

Files changed

File	Change
Cargo.toml / Cargo.lock	add dotenvy = "0.15"
src/main.rs	loadarse failure
.env.example	new, documents all env vars
.gitignore	fix .env from being tracked
README.md	bitcoind prereq, .env onboarding, step fixes

Test plan

• cargo build and cargo build --features rpc succeed without new warnings
• cp .env.example .env, set NETWORK=regtest, run bdk-cli wallet -w <name> balance without -n and confirm the network resolves correctly
• put an invalid line in .env and confirm a warn! log appears and the app still starts
• git check-ignore -v .env returns .gitignore:.env
• walk the regtest steps in README.md end to end

Checklists

• I've signed all my commits
• I followed the [s://github.com/bitcoindevkit/bdk-cli/blob/master/CONTRIBUTING.md)
• I ran cargo fmt before committing

New Features:

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

[notmandatory]

NACK. This project already has a config file system so I don't see any need for this change.

[tvpeter]

The project already uses toml files for managing configs.