add Go sidecar and CLI skeleton

cmd/clide/main.go is the single entry; the same binary will later
switch into sidecar-daemon mode via --daemon. internal/cli holds the
dispatch (stdlib flag for now, Cobra arrives with the first real
subcommand in Tier 2). internal/diag mirrors pql's exit-code contract
so the two tools share one mental model for callers. internal/version
exposes the build-info struct that the Makefile's -ldflags -X targets
stamp; Info() is wired to --version so the binary reports both the
declared project.yaml version and the commit it was built from.

The daemon, pty, proc, git, ipc, and pql sub-packages ship as doc.go
stubs naming the subsystem they will own. They keep the layout legible
before the code lands so tier work has a home on arrival.

Module path targets Gitea (git.schweitz.net/jpmschweitzer/clide/sidecar),
matching where the repo lives. Go module resolution works if the host
serves the meta tags; a vanity import path can be added later if
needed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

sidecar/internal/diag/diag.go

@@ -0,0 +1,58 @@
+// Package diag defines the exit-code contract and emits structured
+// diagnostics to stderr as line-delimited JSON.
+//
+// Mirrors pql's diag contract so Claude (and any other tool that
+// consumes both CLIs) sees a single shape.
+package diag
+
+import (
+	"encoding/json"
+	"io"
+	"os"
+)
+
+// Exit codes. Same numeric space as pql's.
+const (
+	OK       = 0  // success
+	NoMatch  = 2  // success with zero matches / nothing to do
+	Usage    = 64 // EX_USAGE — bad CLI flag or missing subcommand
+	DataErr  = 65 // EX_DATAERR — malformed request
+	NoInput  = 66 // EX_NOINPUT — missing required resource (repo, socket, file)
+	Unavail  = 69 // EX_UNAVAILABLE — sidecar unreachable / subsystem down
+	Software = 70 // EX_SOFTWARE — internal error
+)
+
+type Level string
+
+const (
+	LevelWarn  Level = "warn"
+	LevelError Level = "error"
+)
+
+// Diagnostic is one entry in the stderr JSON-per-line stream.
+type Diagnostic struct {
+	Level Level  `json:"level"`
+	Code  string `json:"code"`
+	Msg   string `json:"msg"`
+	Hint  string `json:"hint,omitempty"`
+}
+
+// Emit writes a diagnostic as one JSON line to w.
+func Emit(w io.Writer, d Diagnostic) {
+	b, err := json.Marshal(d)
+	if err != nil {
+		_, _ = io.WriteString(w, `{"level":"error","code":"diag.marshal","msg":"failed to marshal diagnostic"}`+"\n")
+		return
+	}
+	_, _ = w.Write(append(b, '\n'))
+}
+
+// Warn emits a warning diagnostic to stderr.
+func Warn(code, msg string) {
+	Emit(os.Stderr, Diagnostic{Level: LevelWarn, Code: code, Msg: msg})
+}
+
+// Error emits an error diagnostic to stderr.
+func Error(code, msg, hint string) {
+	Emit(os.Stderr, Diagnostic{Level: LevelError, Code: code, Msg: msg, Hint: hint})
+}