fix(config): make every config key environment-overridable

[jamesainslie]

Summary

Fixes #86.

DOT_ env overrides were driven by four hand-lists that had to agree (the BindEnv whitelist, the per-section env readers, the zero-value merge functions, and the struct itself) and did not:

• Keys outside the lists were silently ignored: dotfile.package_name_mapping (the reported key), symlinks.backup_dir, output.table_style, and the entire update and network sections.
• The sparse-overlay merge only propagated non-zero values, so an env boolean set to false could never override a file value of true. DOT_SYMLINKS_FOLDING=false and DOT_DOTFILE_TRANSLATE=false were also silent no-ops even though they were bound and read.

Changes

• Env bindings derive from the config struct via reflection (tolerant of untagged or non-struct fields), so a new key cannot ship unbound.
• Overrides apply directly onto the loaded config under viper IsSet, replacing the sparse-config merge; explicit false now survives. The env-only mergeConfigs path is deleted; the flags path is untouched (its residual issues are filed as Loader.LoadWithFlags is production-dead and its flag merge cannot represent explicit false #97).
• Path-typed env values expand ~/$VAR through the same ordered pathFields list ExpandPaths uses (shared list, deterministic error attribution).
• List env values are comma-separated, matching the documented contract and the config set convention; viper alone would whitespace-split, turning the documented DOT_IGNORE_PATTERNS="*.log,*.tmp" example into one bogus pattern.
• Docs updated: the stale partial binding list in 04-configuration.md and the "not every key is bound" caveat in 08-troubleshooting.md now state the uniform rule.

Tests

• TestLoader_EnvOverridesEveryConfigKey: reflection walk over every struct key, one valid non-default sample each (vacuous samples rejected), proving each env form lands. Mutation-checked: removing keys from the apply maps fails exactly those subtests.
• TestLoader_EnvFalseOverridesFileTrue: the exact issue repro shape.
• TestLoader_EnvListValuesAreCommaSeparated.

Verified end to end with a built binary: DOT_DOTFILE_PACKAGE_NAME_MAPPING=false now overrides both the default and an explicit file true. Full suite green with race and coverage; golangci-lint clean.