feat: add/reorder package readme

[diegomayorga-dept]

Title*

docs: replace package-level README stubs (DOCS-07)

Type of Change*

• New feature
• Bug fix
• Documentation update
• Refactoring
• Hotfix
• Security patch
• UI/UX improvement

Description

The three package-level READMEs (packages/back-end, packages/front-end, packages/shared-lib) were each a 1-line # replace this placeholder, leaving a new contributor opening any package with zero orientation. This replaces each stub with a concise (~30-line) overview following one shared structure: what the package is → key directories → running locally → where to look next.

Each README links into the in-repo docs/ structure (created in DOCS-04) rather than duplicating content:

• back-end — CDK + Lambda API; directory map; Lambda route/handler conventions; links to docs/development/local-backend-dev.md and docs/development/contributing.md.
• front-end — Nuxt 3 / Vue 3 app; directory map; the layered Pages → Stores → Composables → Repository flow described inline; pnpm dev to run.
• shared-lib — shared types/schemas/utils; how the nf-tower/Seqera types are generated (nftower-spec-to-zod); where the shared HttpError subtypes live.

Design note: the ticket suggested linking into CLAUDE.md, but CLAUDE.md is not tracked in this repo (it lives at the workspace root), so those links would render broken on GitHub. Architecture/convention guidance is therefore described briefly inline and links point only at in-repo docs/ targets.

Relates to DOCS-07 (EGV-169). Depends on DOCS-04 (docs restructure, already merged) so the docs/ links resolve.

Testing*

Documentation-only change — no application code touched, so no automated tests apply. Verified manually:

• Each README is 30–34 lines (within the ticket's 30–60 target, under the 100 cap).
• All relative links resolve to existing in-repo files: docs/README.md, docs/development/contributing.md, docs/development/local-backend-dev.md, packages/shared-lib/src/app/utils/HttpError.ts, and the nf-tower spec.
• No CLAUDE.md references remain in any package README.
• Markdown tables and links render correctly in GitHub preview.

Impact

Documentation only. No runtime, build, dependency, or behavioural changes. Improves contributor onboarding by giving each package a clear entry point into the docs/ structure.

Additional Information

Follow-up worth a separate ticket: CLAUDE.md and ARCHITECTURE.md are not tracked in this repo, so deeper architecture links currently have no in-repo target. Committing them (or an in-repo architecture doc) would let these READMEs link to canonical convention/architecture references.

Checklist*

• No new errors or warnings have been introduced.
• All tests pass successfully and new tests added as necessary.
• Documentation has been updated accordingly.
• Code adheres to the coding and style guidelines of the project.
• Code has been commented in particularly hard-to-understand areas.