feat(mcp): derive env vars from the framework's real struct tags (#3)

Minor to v1.1.0. Make the whole environment-variable surface derive from the
indexed component-config tags instead of a hand-maintained list that drifts.
Also folds in the scaffold-symbol fixes staged as v1.0.1 (never tagged); the
generated scaffold compiles clean against einherjar v1.0.0.

internal/envspec (new):
- Parse index struct tags into env vars {name, module, struct, field, required,
  default}. One source of truth: ParseTag, ForModule, FindStruct, All, KnownNames.

internal/tools:
- get_config_env: list the real env vars a component config reads (or all).
- check_env: flag unknown EINHERJAR_* names, required vars missing for the
  composed modules, and dead vars (set for an uncomposed module).
- get_scaffold: .env.example is now DERIVED from the index — required vars
  uncommented with a dev value, defaulted vars commented with their default.
  The scaffold now composes logz.Config (Log logz.Config) so EINHERJAR_LOG_*
  are live and documented, not hardcoded/ignored (log format is env-driven).
- validate_snippet: inject the real env-var name set into the rules package.

internal/index/builtins (wire conventions):
- Route the incremental "compose a component later" flow to get_config_env /
  check_env; distinguish framework EINHERJAR_* from app-owned APP_* (JWT).
- Compose logz.Config in the config + Run() examples, to match the scaffold.

internal/rules:
- config.unknown-env-var (twelfth rule): reject an env:"EINHERJAR_*" struct tag
  the framework doesn't declare. No-op until the server injects the name set, so
  it never fires on incomplete knowledge.

Fixed (was v1.0.1): scaffold health hook + wire builtin used logz.Logger (real:
contracts/logging.Logger) and postgres.Component (hooks take Provider); env tags
were EINHERJAR_SERVER_ADDR / EINHERJAR_PG_DATABASE (real: _HOST/_PORT / _PG_NAME).

Tests: envspec unit tests; env tools against the real data/index.json; the rule.
Verified by generating the scaffold, building it against local einherjar v1.0.0
(exit 0), and runtime-loading the composed logz.Config (EINHERJAR_LOG_LEVEL=DEBUG
-> slog.LevelDebug, EINHERJAR_LOG_JSON=true). Version bumped to v1.1.0 (badge +
serverVersion). No dependency changes.

Reviewed-on: #3
Co-authored-by: Rene Nochebuena Guerrero <rene@nochebuena.dev>
Co-committed-by: Rene Nochebuena Guerrero <rene@nochebuena.dev>

internal/tools/get_config_env.go

@@ -0,0 +1,60 @@
+package tools
+
+import (
+	"context"
+	"strings"
+
+	"code.nochebuena.dev/einherjar/mcp/internal/envspec"
+	"code.nochebuena.dev/einherjar/mcp/internal/index"
+	"github.com/modelcontextprotocol/go-sdk/mcp"
+)
+
+type getConfigEnvInput struct {
+	Module string `json:"module,omitempty" jsonschema:"restrict to one module's env vars, e.g. db-postgres or web. Omit to return every framework env var."`
+}
+
+type getConfigEnvOutput struct {
+	Vars    []envspec.Var `json:"vars"`
+	Count   int           `json:"count"`
+	Summary string        `json:"summary"`
+}
+
+func registerGetConfigEnv(s *mcp.Server, idx *index.Index) {
+	mcp.AddTool(s, &mcp.Tool{
+		Name: "get_config_env",
+		Description: "List the REAL environment variables an Einherjar component config reads, derived from the framework's struct tags (never hand-maintained). " +
+			"Each entry gives the var name, the declaring module/struct/field, whether it is required, and its default. " +
+			"Use this before composing a global Config or writing a .env — it is the source of truth for which EINHERJAR_* vars exist. " +
+			"Pass a module (e.g. db-postgres, web, core, storage-minio) to scope it; omit to get all.",
+	}, func(ctx context.Context, req *mcp.CallToolRequest, args getConfigEnvInput) (*mcp.CallToolResult, getConfigEnvOutput, error) {
+		module := strings.TrimSpace(args.Module)
+		var vars []envspec.Var
+		if module == "" {
+			vars = envspec.All(idx)
+		} else {
+			if idx.FindModule(module) == nil {
+				return errorResult("module not found: " + module), getConfigEnvOutput{}, nil
+			}
+			vars = envspec.ForModule(idx, module)
+		}
+		if vars == nil {
+			vars = []envspec.Var{}
+		}
+		out := getConfigEnvOutput{Vars: vars, Count: len(vars), Summary: summariseEnv(vars, module)}
+		return jsonText(out), out, nil
+	})
+}
+
+func summariseEnv(vars []envspec.Var, module string) string {
+	var required int
+	for _, v := range vars {
+		if v.Required {
+			required++
+		}
+	}
+	scope := "the framework"
+	if module != "" {
+		scope = module
+	}
+	return itoa(len(vars)) + " env var(s) for " + scope + ", " + itoa(required) + " required."
+}