Add pony-lint and fix all style errors

[SeanTAllen]

Adds a pony-lint GitHub Actions workflow and Makefile lint target, then fixes all style errors across the codebase.

Style fixes applied:

• acronym-casing: Json -> JSON throughout (JSONDoc, JSONType, JSONArray, JSONObject, JSONError, _JSONPrint)
• member-naming: camelCase -> snake_case (resultReceiver, reportError, actionResult, matchIndex, allMatch, newCells, cellsFromDeps, fromCell, setName, fieldId, and others)
• line-length: wrapped lines exceeding 80 columns
• public-docstring: added docstrings to all public types and methods
• package-docstring: created package docstring files for packages that lacked them (cmd.pony, corral.pony, mort.pony, util.pony, test.pony, integration.pony, utils.pony, readme.pony)
• assignment-indent: moved multiline assignment RHS to start on the line after =
• match-case-indent: aligned | with match keyword
• call-argument-format: when call args span multiple lines, put each on its own line
• array-literal-format: added space after [ in multiline arrays
• blank-lines: exactly 1 blank line between entities
• comment-spacing: single space after //
• dot-spacing: space after .> operator
• control-structure-alignment: aligned else/end with opening keyword
• indentation-size: fixed indentation to multiples of 2 spaces
• lambda-spacing: space before } in single-line lambdas
• prefer-chaining: replaced intermediate variables with .> chaining where safe
• exhaustive-match: added \exhaustive\ annotation
• docstring-format: fixed docstring formatting

Remaining style/file-naming errors (not addressed in this PR to keep the diff reviewable):

• corral/archive/decoder.pony -> archive_decoder.pony
• corral/archive/encoder.pony -> archive_encoder.pony
• corral/bundle/_flatttened.pony -> _flattened.pony
• corral/cmd/repo.pony -> repo_for_dep.pony
• corral/cmd/result_receiver.pony -> cmd_result_receiver.pony
• corral/cmd/script.pony -> post_fetch_script.pony
• corral/logger/_test.pony -> _string_logger_test.pony
• corral/logger/logger.pony -> log_formatter.pony
• corral/semver/solver/cell.pony -> _cell.pony
• corral/semver/solver/conflict_snapshot.pony -> _conflict_snapshot.pony
• corral/test/util.pony -> checker.pony
• corral/vcs/bzr.pony -> bzr_vcs.pony
• corral/vcs/hg.pony -> hg_vcs.pony
• corral/vcs/svn.pony -> svn_vcs.pony
• corral/vcs/vcs.pony -> tag_list_receiver.pony

Also remaining: lint/ast-error entries from relative imports (expected and unavoidable).