chore: clean up pre horizon code

[tmigone]

1. Removed escrow_accounts_v1() dead code from monitor crate:

• Deleted escrow_accounts_v1() function (~12 lines)
• Deleted get_escrow_accounts_v1() function (~69 lines)
• Deleted test_current_accounts() test (~46 lines)
• Removed unused imports

2. Removed is_horizon_active() and simplified call sites:

• Deleted crates/monitor/src/horizon_detection.rs (~53 lines) - this always returned Ok(true)
• Updated crates/tap-agent/src/agent.rs - removed the horizon check wrapper
• Updated crates/service/src/service.rs - removed the horizon check wrapper
• Updated crates/monitor/src/lib.rs - removed module export

3. Updated outdated V1 comments and docs:

• crates/service/src/tap.rs - Simplified receipt types documentation to only mention Horizon (V2)
• crates/service/tests/router_test.rs - Removed mention of escrow subgraph (v1) in comment
• crates/service/src/tap/receipt_store.rs - Renamed test from v1_and_v2_are_processed_successfully to
  receipts_are_processed_successfully
• crates/monitor/src/escrow_accounts.rs - Updated test comment to just "Verify V2 escrow accounts are loaded correctly"

[coveralls]

Coverage Report for CI Build 24677315757

Coverage increased (+0.1%) to 71.383%

Details

• Coverage increased (+0.1%) from the base build.
• Patch coverage: No coverable lines changed in this PR.
• 2 coverage regressions across 1 file.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

2 previously-covered lines in 1 file lost coverage.

File	Lines Losing Coverage	Coverage
crates/service/src/service.rs	2	0.0%

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Coverage Stats

Relevant Lines:	14778
Covered Lines:	10549
Line Coverage:	71.38%
Coverage Strength:	113.45 hits per line

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💛 - Coveralls

The base branch was changed.