feat: Go rewrite M1 — skeleton, tooling, and hello server

Stand up the Go project alongside the Python backend so both run
independently during migration. `make web-go` builds and serves on :8080;
`make web-py` (alias: `make web`) keeps the Python side on :5001.

- go/: new module `fuj-management/go` (Go 1.26)
  - cmd/fuj: stdlib-flag dispatcher; `server` + `version` work,
    fees/reconcile/sync/infer stubbed for M2/M4
  - internal/config: env loader mirroring scripts/config.py
  - internal/logging: slog setup, level taken from config
  - internal/web: net/http ServeMux + request-timer middleware
  - build/Dockerfile: golang:1.26 → alpine:3 multi-stage image
  - .golangci.yml: govet, staticcheck, errcheck, gofumpt, unused
- Makefile: web→web-py alias; go-build/go-test/go-run/go-lint/web-go
- CI: parallel build-go job in .gitea/workflows/build.yaml (<tag>-go image)
- docs/plans/: M1 kickoff plan + progress tracker (M1 complete)
- .claude/settings.json: gofumpt + golangci-lint permissions

Gate: make go-build ✓  make go-lint ✓  make go-test ✓  curl :8080 ✓

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

CLAUDE.md

@@ -108,3 +108,13 @@ Maintain a running changelog in `CHANGELOG.md` at the repo root. After every sig
 ```
 
 Get the timestamp with `date "+%Y-%m-%d %H:%M %Z"`. Skip trivial edits (typos, formatting, comment tweaks); only log changes a future reader would care about.
+
+## Plans
+
+When Claude Code's plan mode is used, save the plan file inside the repo at
+`docs/plans/YYYY-MM-DD-HHMM-<slug>.md` instead of the default `~/.claude/plans/`
+location. Get the timestamp with `date "+%Y-%m-%d-%H%M"` (matches the changelog
+convention). The `<slug>` should be a short kebab-case summary of the plan's topic.
+
+Create the `docs/plans/` directory on first use. Plan files are committed to the
+repo so other contributors can review historical decisions.