fix(ci): repair failing lint and test jobs

[hahwul]

Why

CI on main has been failing on both the lint and tests jobs since the refactor in 00b1ee8.

Root cause

• tests (ubuntu + macos): spec/spec_helper.cr required ../lib/webmock/src/webmock, described as "vendored... committed in tree". But lib/ is gitignored, webmock was never committed, and it wasn't a declared dependency — so CI's clean shards install only fetched ameba and spec compilation failed with can't find file '../lib/webmock/...'.
• lint: the same commit enforced ameba by removing continue-on-error: true, but never cleaned up the ~184 pre-existing violations and added no config.

What changed

Tests

• Add webmock as a proper development_dependency (shard.yml / shard.lock).
• Require it normally in spec_helper.cr; drop the misleading comment and the unused require_webmock! helper.

Lint

• Add .ameba.yml that disables only rules conflicting with deliberate project conventions, each with a rationale:
  • Naming/VariableNames — camelCase vars (createTime, gitPatch, …) mirror the Jules REST API JSON fields 1:1.
  • Naming/BlockParameterName — short block params used idiomatically.
  • Metrics/CyclomaticComplexity — CLI dispatcher / flag parsers are inherently branch-heavy.
  • Lint/NotNil — flag values are captured in OptionParser closures, where Crystal flow typing can't narrow and not_nil! is the idiomatic guard (plus spec fixtures).
• Fix the remaining real issues in code (not config): RedundantBegin, VerboseBlock, negated unless, lines.each → each_line, .sort → .sort!, the x.not_nil!.foo if x pattern → if x = ..., and getter quit_early → getter? quit_early?.

Robustness (prevents recurrence)

• Pin the CI lint job to the ameba version locked in shard.lock (bin/ameba) instead of crystal-ameba/github-action@master. The @master action always runs bleeding-edge ameba, so a new upstream rule (e.g. the Signal::INT.trap one that fires on master but not local 1.6.4) could break CI with no code change. Now local just lint == CI.

Verification (local)

shards build                  → OK
bin/ameba                     → 44 inspected, 0 failures
crystal spec                  → 97 examples, 0 failures
crystal tool format --check   → OK