Add fish shell completion

[montionugera]

What and why

Adds static fish shell completions (completions/btc-toolkit.fish) mirroring completions/btc-toolkit.zsh for all 11 subcommands and their supported flags, extends tests/test_completions.py to ensure fish completions stay synchronized with build_parser(), and documents fish completion in README.md and CHANGELOG.md.

Closes #62

Checklist

• Standard library only: no new runtime dependency
• Tests added or updated in tests/ (unittest, API calls mocked, runs offline)
• python -m coverage run -m unittest discover -s tests -t . then python -m coverage report (90% or more)
• ruff check btc_toolkit/ tests/ fuzz/ .github/scripts/ and mypy --strict btc_toolkit/ pass
• A new parser of untrusted data has a fuzz target in fuzz/fuzz_parsers.py
• --json keys only added, never renamed or removed (docs/json-schema.md)
• Every txid, address or on-chain fact in docs or tests can be checked against the chain
• A user-visible change has a line under [Unreleased] in CHANGELOG.md

[devdavidejesus]

Thanks a lot for this, and welcome! It's the project's first outside contribution.

I tested it in a real fish shell (3.7.0): all 11 commands, their flags and the networks complete correctly, and the tests pass. Two small changes before I merge:

1. -r lets fish suggest file names for the option's value. That fits --file, but not the options that take a value, where it now lists the current directory (bash and zsh complete nothing there). Could you use -x for these four?

   -l api-url -x
   -l timeout -x
   -l limit -x
   -l txid -x
   

2. The Homebrew formula only installs the bash and zsh scripts for now, so in the README please keep that sentence true, e.g.: "The Homebrew formula installs the bash and zsh ones for you." I'll add fish to the formula with the next release.

Then I'll merge.

[montionugera]

Thanks for the review and testing, @devdavidejesus!

Updated both points:

1. Changed --api-url, --timeout, --limit, and --txid in completions/btc-toolkit.fish to use -x instead of -r.
2. Updated README.md to keep "The Homebrew formula installs the bash and zsh ones for you."