feat: shipnode config show --raw / --diff to surface dropped fields

[devalade]

What to build

shipnode config show currently prints only the resolved config (post-assembleConfig). When a builder method has no effect at runtime (see #7 for .aliases() / .worker() in 2.5.1), the operator has no signal — the field just isn't there. Add flags that expose the assembleConfig contract:

• --raw — print the raw builder-chain output before assembleConfig runs
• --diff — print a side-by-side (or unified) diff between raw and resolved, with dropped keys highlighted in red

The point is diagnostic: any time a user says "but I put X in my config", config show --diff tells them in one command whether X survived the assembly step.

Acceptance criteria

• config show --raw prints the pre-assembly config as JSON
• config show --diff prints a diff of raw → resolved, dropped keys visually distinct
• For a fixture config that uses .aliases(), --diff shows the aliases key as dropped (until bug: .aliases() and .worker() silently dropped by assembleConfig #7 is fixed; then it stays present)
• Both flags respect --config <path>
• --json still works on the default show output (unchanged behavior when neither new flag is set)

Blocked by

None - can start immediately