Merge defaults across successive tasks

[johnlettman]

Recursively merge Dotbot defaults instead of replacing the existing mapping, preserving unrelated plugin settings while allowing later values to override specific nested options.

The change makes successive defaults directives merge recursively instead of replacing the entire defaults mapping.

Example

Starting from the following example:

- defaults:
    shell:
      quiet: true
    link:
      create: true
      relink: true
    sync:
      stdout: true
      stderr: true
      options: ["--safe-links"]

- if:
    cond: '[ -v DOT_NO_AUR_PROMPT ]'
    met:
      - defaults:
          paru:
            interactive: false
    unmet:
      - defaults:
          paru:
            interactive: true

When the met branch runs, Dotbot now produces defaults equivalent to:

shell:
  quiet: true
link:
  create: true
  relink: true
sync:
  stdout: true
  stderr: true
  options: ["--safe-links"]
paru:
  interactive: false

Previously, adding the paru defaults replaced the entire mapping, silently discarding the existing shell, link, and sync settings. Recursive merging preserves those settings while allowing the conditional branch to add or override only paru.interactive.