feat(core): initial implementation — launcher, logz, xerrors, valid

Introduces `code.nochebuena.dev/einherjar/core` — the foundational implementation
module of the Einherjar framework. Provides four sub-packages that together cover
every service's baseline needs: lifecycle management, structured logging, typed
errors, and struct validation.

- launcher: Launcher interface — three-phase managed lifecycle (OnInit → BeforeStart
  hooks → OnStart → OS signal wait → OnStop in reverse). Accepts
  lifecycle.Component and logging.Logger from contracts. Prints an ASCII art banner
  at startup (EINHERJAR_BANNER=off to suppress). Banner includes core version via
  runtime/debug.ReadBuildInfo() and a loaded-module list for every registered
  component that implements observability.Identifiable. Config struct with
  EINHERJAR_COMPONENT_STOP_TIMEOUT env tag (caarlos0/env syntax, default 15s).

- logz: Logger implementation backed by log/slog. Returns contracts/logging.Logger.
  Detects errs.CodedError and errs.ContextualError (from contracts/errs) to enrich
  log records automatically — replaces the private duck-typed bridge from micro-lib.
  Context helpers: WithRequestID, WithField, WithFields, GetRequestID. Config struct
  with EINHERJAR_LOG_LEVEL (default INFO) and EINHERJAR_LOG_JSON (default false) env
  tags (caarlos0/env syntax); programmatic-only fields StaticArgs and Writer carry no
  tags.

- xerrors: Typed error codes with context enrichment. Complete gRPC canonical set
  (16 codes) plus HTTP 410 ErrGone. Adds ErrOutOfRange, ErrAborted, ErrDataLoss
  over micro-lib. One convenience constructor per code. *Err declares compile-time
  satisfaction of errs.CodedError and errs.ContextualError.

- valid: Struct validation wrapping go-playground/validator/v10. Validator interface
  + MessageProvider interface with full built-in tag coverage (~150 tags) in both
  DefaultMessages (English) and SpanishMessages (Spanish). Backend fully hidden;
  returns *xerrors.Err with ErrInvalidInput or ErrInternal. FieldLevel interface
  abstracts the backend's field-level access for custom validators.
  WithCustomValidator registers custom validation tags at construction time;
  OverrideProvider chains a tag→handler map with a fallback MessageProvider for
  custom tag messages without re-implementing built-ins.

Compliance test enforces CT-6 (at most one exported TypeSpec per file via AST) and
verifies behavioural correctness of all four sub-packages, including custom validator
registration and OverrideProvider composition. Compile-time var _ assertions prove
interface satisfaction.

docs: ADR-001 (core module composition), ADR-002 (logz contracts/errs adoption),
ADR-003 (Config naming convention and caarlos0/env tag standard)

.gitea/CODEOWNERS

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+* @einherjar/CoreDevelopers @einherjar/Agents

.gitea/pull_request_template.md

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+## Summary
+
+<!-- One or two sentences: what does this PR do and why? -->
+
+---
+
+## Type of change
+
+- [ ] Bug fix — non-breaking change that resolves an issue
+- [ ] New feature — non-breaking addition of functionality
+- [ ] Breaking change — alters existing behavior or public API
+- [ ] Documentation update
+- [ ] Refactor — no functional change, no new API surface
+- [ ] Test improvement
+
+---
+
+## Description
+
+<!--
+Provide enough context for a reviewer who was not in the room:
+- What problem does this solve?
+- What approach did you choose, and why?
+- Were there alternatives you considered and rejected?
+- Any known limitations or follow-up work?
+-->
+
+---
+
+## Testing
+
+- [ ] I added or updated tests that cover my changes
+- [ ] All tests pass locally — `go test ./...`
+- [ ] No formatting issues — `gofmt -l .` produces no output
+- [ ] No vet warnings — `go vet ./...` is clean
+
+---
+
+## Checklist
+
+- [ ] At most one exported type per non-test `.go` file (CT-6)
+- [ ] No new external dependencies added without prior discussion in an issue
+- [ ] Public API changes are reflected in `CHANGELOG.md`
+- [ ] Breaking changes include a migration note in the PR description above
+
+---
+
+## Contributor License Agreement
+
+> **This PR will not be merged until the CLA comment is present.**
+
+Before a Maintainer reviews your code, you must post the following text **as a comment on this PR** — not here in the description. PR description checkboxes can be silently toggled by anyone; a comment is a timestamped, author-attributed record that cannot be quietly removed.
+
+**Copy and post this exact text as a PR comment:**
+
+---
+
+> I have read the Einherjar Contributor License Agreement (CLA.md) and I agree to all its terms.
+> I confirm this Contribution is my original work. I grant the Maintainers the rights described
+> therein, including the right to relicense, and I retain ownership of my copyright.
+> This agreement covers all future Contributions I submit to any Einherjar repository under
+> this account.
+
+---
+
+First time contributing? Read [CLA.md](../CLA.md) for the full agreement before posting the comment.
+
+If you are contributing on behalf of a company, an authorized representative of that company must post the comment.
+
+<!-- Thank you for contributing to Einherjar. For those who come after. -->