feat: add - prefix to ignore non-zero exit codes in Opsfile commands (Issue #20)

[seanseannery]

Key Changes

• Add IgnoreError bool field to ResolvedLine in internal/command_resolver.go
• Replace single @ prefix check in Resolve() with a loop that handles @, -, -@, @- in any order (at most one of each consumed)
• Add IgnoreError check in internal/executor.go — uses errors.As(err, &exitErr) with *exec.ExitError so only non-zero exit codes are ignored; system-level errors (shell not found, permission denied) still propagate
• Add redeploy example command to examples/Opsfile demonstrating - and -@ prefixes
• Add design doc at docs/feat-dash-prefix-silent-failure.md and test plan at docs/testplans/testplan-dash-prefix-silent-failure.md

Why do we need this?

Closes #20. Ops currently stops on the first shell line that returns a non-zero exit code. Some operations (e.g. docker stop, rm -f, killall) may legitimately fail without indicating a real problem. This adds Makefile-style - prefix support so users can mark individual lines as failure-tolerant, allowing execution to continue to the next line.

New modules or other dependencies introduced

None. Uses errors.As from the standard library (already imported).

How was this tested?

• 20 new unit tests in internal/command_resolver_test.go covering: - stripping, combined -@/@-, -- (double-dash), -@-/@-@ edge cases, dash in middle of line, dash in variable values, multi-line continuation, whitespace after dash
• 9 new unit tests in internal/executor_test.go covering: ignored exit codes, system errors still propagate (invalid shell test), combined IgnoreError+Silent, all-lines IgnoreError, exit code 127 ignored
• internal/opsfile_parser_test.go updated for new redeploy example command
• All existing @ prefix tests (20+) pass with the refactored loop-based prefix stripping
• make lint and make test pass cleanly