default-config: Bind swap to h/j/k/l in service mode

[freqyfreqy]

Changes

Adds four bindings to the default config's service mode:

h = 'swap left'
j = 'swap down'
k = 'swap up'
l = 'swap right'

Rationale is in the commit message: swap is under-discoverable (users only learn move and its tree-restructuring surprises); service mode already hosts join-with, so it becomes the complete restructure hub; bare h/j/k/l collide with no existing default binding; no mode main tail so swaps chain, esc exits as always.

Testing

• ./test.sh — ✅ All tests have passed successfully (requires bash 5, installed via Homebrew to run it; the repo's own script/setup.sh gate)
• Includes testParseDefaultConfig (default config parses with 0 errors / 0 warnings), lint.sh, generate.sh + check-uncommitted-files.sh (TOML-only change; no generated-file drift)

PR checklist

• Explain your changes in the relevant commit messages rather than in the PR description. The PR description must not contain more information than the commit messages (except for images and other media).
• Each commit must explain what/why/how and motivation in its description. https://cbea.ms/git-commit/
• Don't forget to link the appropriate issues/discussions in commit messages (if applicable). (searched issues/PRs/discussions; no prior discussion found)
• Each commit must be an atomic change (a PR may contain several commits). Don't introduce new functional changes together with refactorings in the same commit.
• ./test.sh exits with zero exit code (suite passes).
• Avoid merge commits, always rebase and force push.

[nikitabobko]

'swap' is under-discoverable in the default config

Thanks for the raised concern, but I am not sure that the problem is legit.

Users can use mouse drag to discover it. Then it's a matter of assigning it in the default config if they prefer swap over move.