feat(obs): wire redacting slog handler into rune-mcp

[couragehong]

문제점: 그동안 obs 패키지는 TODO 투성이 stub. 즉

• Vault 토큰이 error message 에 그대로 노출
• envector API key 가 attr 으로 stderr 직출
• ~/.rune/logs/rune-mcp.log 도 동일 — dev 가 화면공유로 tail 하면 그대로 새는 구조

또한 obs.NewHandler 는 어디서도 안 쓰이고 있어서 현재 default slog 가 raw text 만 출력.

변경 — 3가지 layer:

(1) filteringHandler 새 타입 — slog.Handler 인터페이스의 4 메서드 구현:

• Handle: Record 의 Message + 모든 string-typed attr 을 redact 통과시킴
• WithAttrs: logger.With(slog.String("api_key", ...)) 도 redact
• WithGroup: 중첩 group attr 도 redact (재귀)
• Enabled: inner handler 에 위임

(2) Redaction regex 두 개 (Python SensitiveFilter 와 1:1 동일):
(sk-|pk-|api|envector_|evt_)[a-zA-Z0-9_-]{10,}
(?i)(token|key|secret|password)["\s:=]+[a-zA-Z0-9_-]{20,}
match 의 첫 8글자만 보존 + *** 추가:

• evt_AABBCCDD11223344 → evt_AABB***
• token: ABCDEFGHIJKLMNOP... → token: A***

prefix 일부는 grep 가능하게 남겨서 incident 조사 시 매칭 가능.

(3) NewRequestID UUID v4 — crypto/rand 기반 36자 UUID. rand.Read 실패하면 panic 대신 req-anon 으로 fallback (broken host RNG 에서도 서버는 살아있게).

(4) main.go wire: stderr 와 file tee (있을 시) 둘 다 obs.NewHandler 거쳐서 redaction 적용. RUNE_MCP_LOG_FILE 안 셋팅한 production 도 redaction 활성화.

internal/obs/slog.go was full of TODOs that left rune-mcp logging unfiltered: a Vault token in an error message, an envector_ API key in an attr, or a labeled secret in a debug message all flowed through to stderr (and the optional file tee) verbatim. This is a real exposure on dev hosts where users tail ~/.rune/logs/rune-mcp.log over screen share, and a worse one for any future production deploy that ships logs off-host.

Implementation

filteringHandler wraps an inner slog.Handler. Each Record's Message plus every string-typed attr (recursing into KindGroup, best-effort on KindAny w/ fmt.Stringer) is rewritten through two regexes ported 1-for-1 from Python's _SensitiveFilter:

(sk-|pk-|api_|envector_|evt_)[a-zA-Z0-9_-]{10,}
(?i)(token|key|secret|password)["\s:=]+[a-zA-Z0-9_-]{20,}

Replacement keeps the first 8 chars of the match plus "", so prefixes like "evt_AABB" stay grep-able for incident investigation while the body is destroyed. Strings ≤8 chars get the full "***" treatment — preserving prefix would leak the secret.

WithAttrs / WithGroup chain through the inner handler so logger.With( slog.String("api_key", ...)) is also caught.

NewRequestID returns RFC 4122 v4 UUIDs from crypto/rand. Falls back to "req-anon" on rand.Read failure rather than panicking — a working log without per-request correlation beats a crashing server. Format

• uniqueness + roundtrip-via-context are all covered.

Wiring

cmd/rune-mcp/main.go now always wraps the chosen TextHandler with obs.NewHandler before SetDefault. Both stderr-only and tee-to-file flows go through redaction; an unset RUNE_MCP_LOG_FILE no longer means "skip filtering."

Tests

Adds slog_test.go covering: each token prefix once, labeled-secret edge cases (JSON quoting, =, " ", short-fail), filteringHandler vs attrs / message / With(), UUID format + uniqueness across 1024 ids

• context round-trip.