Normalize output-format parsing and rendering #172

Parent: #171

Problem

Commands that already expose -f/--format do not consistently honor the requested format or distinguish explicit values from defaults.

Verified cases:

• plugin list -f yaml prints the human table because only json is handled specially (src/cli.ts:3169-3226).
• Empty plugin list -f json prints human guidance instead of valid JSON (src/cli.ts:3169-3189).
• convention-audit accepts arbitrary values but only JSON/YAML use the shared renderer (src/cli.ts:918-938).
• Unknown formats silently fall through to table output (src/command-surface.ts:249-252, src/output.ts:43-57).
• list, plugin catalog/search commands, and external list omit fmtExplicit, so explicit -f table can become YAML on non-TTY stdout.
• skills list has the opposite bug: its default table is always treated as explicit (src/cli.ts:857-869).
• Plugin catalog/search help advertises fewer formats than the renderer accepts.

Scope

• Validate format names and return a usage error for unsupported values.
• Use one shared supported-format definition and help description.
• Derive fmtExplicit from Commander’s option-value source everywhere.
• Ensure explicit -f table remains table and implicit non-TTY output follows the documented default.
• Make empty and non-empty results obey the same format contract.
• Either implement every advertised format or reject it explicitly.

Acceptance criteria

• plugin list -f yaml emits YAML or a clear unsupported-format error, never a table.
• Empty plugin list -f json emits valid JSON.
• Unknown formats fail with a usage error.
• Explicit and implicit table behavior is correct on non-TTY stdout.
• Format help and parsing use the same source of truth.
• Tests cover JSON, YAML, explicit table, implicit non-TTY output, empty results, and invalid formats.