contrib: use BGL binaries in utility scripts

[pinehill99]

Refs #39, #81.

Summary:

• replace stale bitcoin-cli and bitcoind invocations in the UTXO snapshot contrib demo with BGL-cli and BGLd
• update the signet miner helper default CLI and debug text to BGL-cli

Validation:

• git diff --check
• bash -n contrib/devtools/test_utxo_snapshots.sh
• python3 -m py_compile contrib/signet/miner
• rg -n 'bitcoin-cli|bitcoind' contrib/devtools/test_utxo_snapshots.sh contrib/signet/miner returned no matches\n\nRuntime execution was not run because the UTXO snapshot demo needs built BGL binaries and a long-running chain sync environment.\n\nBounty submission for BGL PR bounty hunt ($10000 overall budget) #39.

[MyTH-zyxeon]

Review-assist note for #39 / #81 / #292:

This looks like a useful utility-script cleanup because it updates actual contrib script invocations, not just surrounding prose. The visible diff switches the UTXO snapshot demo and signet miner helper from inherited Bitcoin binary names to BGL binary names while keeping the long-running runtime path explicitly out of scope.

Acceptance checks I would keep explicit before merge/payout:

• Confirm the canonical built binary names are exactly ./src/BGLd and ./src/BGL-cli on the supported build/release targets. If lowercase wrappers are also shipped, decide whether these contrib helpers should use marketed names or the actual most portable executable names.
• For contrib/devtools/test_utxo_snapshots.sh, the no-runtime validation is understandable because the demo needs built binaries and a long chain-sync flow, but maintainers should still confirm a syntax-only pass plus a command-name grep is sufficient for this BGL PR bounty hunt ($10000 overall budget) #39/BGL (Bitgesell) Bounty/Improvement Program ($50000 budget) #81 slice.
• For contrib/signet/miner, python3 -m py_compile contrib/signet/miner is a good minimum; a mocked --cli smoke would further prove the default command string and debug log path changed without touching signet behavior.
• Check whether the helper function name bitcoin_cli is intentionally kept as an internal implementation detail. If maintainers prefer complete terminology cleanup, it could be a follow-up, but renaming it in this PR may widen the diff unnecessarily.

I did not run builds, install dependencies, start nodes, use wallet keys, or perform any live-chain action. This is a maintainer-facing review checklist to help decide whether #292 should be accepted as a focused utility-script terminology fix.

[pinehill99]

Rechecked the current PR head after the review-assist note.

Evidence from this checkout:

• PR contrib: use BGL binaries in utility scripts #292 is still open, non-draft, and CLEAN/mergeable at head 01d35e5.
• The local checkout is clean.
• Build metadata confirms the canonical names: configure.ac has BGL_DAEMON_NAME=BGLd and BGL_CLI_NAME=BGL-cli, and src/Makefile.am adds BGLd and BGL-cli to bin_PROGRAMS.
• git diff --check origin/master...HEAD passed.
• bash -n contrib/devtools/test_utxo_snapshots.sh passed.
• Python syntax compile for contrib/signet/miner passed with the pyc output redirected to /tmp.
• rg over the two touched files found no remaining bitcoind or bitcoin-cli strings.

I did not run the long chain-sync or miner runtime path locally, so that remains the maintainer/runtime acceptance gate for this #39/#81 slice.