feat(config): strict, self-explanatory config.json parsing

[Emasoft]

A typo in config.json used to buy you either silence or a pile of silently-applied defaults. Now every parse problem is named, positioned, and fatal — and booleans speak human.

FlexBool — the full on/off synonym table

Booleans accept true/on/enabled/enable/active/activated/yes/y/1 vs false/off/disabled/disable/no/not/n/0 — case-insensitive, whitespace-tolerant, with bare 1/0 numbers allowed. Anything else is an error that lists what's accepted.

Every config error is self-explanatory

Each error renders as error in <full path> at line L, column C: <detail> — for syntax errors, unknown keys (with a Levenshtein did-you-mean suggestion), wrong-typed values, invalid enum values, and invalid boolean synonyms alike.

Config errors are fatal

A broken config exits 1 before the TUI starts. No silent degradation to defaults while you believe everything's fine. A missing file still boots a fresh install, unchanged.

Parser hardening

• trailing data (concatenated top-level JSON values) is rejected instead of silently dropped — a partially-applied config is worse than an error
• UTF-8 BOM is tolerated: stripped, with offsets kept editor-visible
• EOF errors anchor to the end of input
• duplicate keys, CRLF column math, nested keys not flagged as unknown, and tri-state null handling are all pinned by tests

docs/config-reference.md

A full reference for this branch's config keys, enforced by a reflection guard test: every documented row must be a real Config json tag and every tag must be documented, so doc and struct can't drift apart.

Scope note: this PR covers the fields that exist on today's main. Sibling PRs extend both the schema and the doc as they add fields.

Testing

$ go test -count=1 -race ./...
ok  	late/cmd/late	3.780s
?  	late/cmd/mcp-run	[no test files]
ok  	late/internal/agent	2.194s
?  	late/internal/assets	[no test files]
ok  	late/internal/client	2.622s
ok  	late/internal/common	4.699s
ok  	late/internal/config	2.716s
ok  	late/internal/executor	9.329s
ok  	late/internal/git	2.348s
ok  	late/internal/mcp	2.945s
ok  	late/internal/orchestrator	2.669s
?  	late/internal/pathutil	[no test files]
ok  	late/internal/plugin	12.131s
ok  	late/internal/session	3.807s
ok  	late/internal/skill	3.926s
ok  	late/internal/tool	2.990s
ok  	late/internal/tool/ast	3.303s
ok  	late/internal/tui	8.322s

exit 0 — all 18 packages green (15 ok, 3 without test files).