feat(flags): add --list/-l flag to display available commands and environments

[seanseannery]

Key Changes

• Add --list / -l CLI flag that prints a summary of all commands and environments defined in the Opsfile, then exits
• Parser now captures # comment descriptions from the first line of a comment block directly above a command declaration
• Parser returns command declaration order and environment appearance order for stable, deterministic output
• --help now appends command listing after flag help (silently skips if no Opsfile found)
• New internal/lister.go module handles formatted output with column-aligned descriptions

Why do we need this?

There is no way to discover available commands without opening the Opsfile manually. Under time pressure (e.g. during incidents), this adds unnecessary friction. The --list flag provides instant command discovery from the terminal.

Closes #19

New modules or other dependencies introduced

• internal/lister.go — new file containing FormatCommandList() for rendering the command/environment listing. Uses only the standard library (fmt, io, strings).
• internal/lister_test.go — 7 table-driven test cases for output formatting

No new external dependencies.

How was this tested?

• Unit tests (parser): 4 new subtests for description extraction (direct comment, blank-line gap, multi-line block first-line capture, no comment), plus command order and environment order tests
• Unit tests (flag parser): 4 new subtests for --list/-l flag parsing including combination with -D
• Unit tests (lister): 7 table-driven tests covering descriptions, no descriptions, mixed, env order preservation, column alignment, single command, and empty map
• Existing tests updated: command_resolver_test.go updated for new ParseOpsFile return signature
• Manual smoke test: ops --list -D ./examples produces correct output against the example Opsfile
• make lint and make test pass (88 tests total)