feat: add force16Color config option

[jojosenthusiast]

Closes #1144

Reproduction

src previously either emitted no colours or 256-colour ANSI SGR sequences such as 38;5;N / 48;5;N. Users with terminal themes that make those 256-colour choices hard to read had to disable colour entirely.

Fix summary

• Add force16Color to ~/src-config.json parsing.
• Apply the setting in both legacy and migrated command paths.
• Remap cmd/src and lib/output 256-colour SGR sequences to terminal-theme 16-colour SGR when enabled.
• Preserve disabled-colour behavior for blanked ansiColors entries.
• Document the config key in the README.

Tests run

• go test -count=1 ./cmd/src
• (cd lib && go test -count=1 ./output)
• gofmt -l cmd/src/cmd_test.go cmd/src/colors_test.go cmd/src/run_migration_compat_test.go cmd/src/main_test.go cmd/src/colors.go cmd/src/cmd.go cmd/src/main.go cmd/src/run_migration_compat.go lib/output/style.go lib/output/style_test.go lib/output/setup_test.go
• Judgment Day: two blind reviewers approved after final fixes.
• Ponytail review: Lean already. Ship.

Risk

Low-to-medium. This touches global CLI colour rendering state, but the default behavior is unchanged unless force16Color is enabled. Regression tests cover config parsing, legacy command dispatch, migrated command dispatch, cmd/src colour remapping, lib/output style remapping, and disabled-colour preservation.

Non-goals

• No new CLI flag.
• No perceptual colour-distance algorithm; the remap only guarantees terminal-theme 16-colour SGR instead of 256-colour SGR.
• No changes to unrelated batch/service behavior.