docs: fix 21 broken doc links in README (closes #58)

[initializ-mk]

Summary

Fixes #58 — the top-level README.md referenced 25 relative .md paths and 21 were 404s. Newcomers clicking "Quick Start", "Architecture", "Skills", "Runtime", "Memory", "Channels", "Egress Security", "Secrets", "Build Signing", "Commands", "Configuration", "Dashboard", "Deployment", "Hooks", "Plugins", or "Command Integration" from the README landed on a GitHub 404 page. That's a serious first-impression bug on the project's most visible doc.

Approach

Option A from the issue — pure link rewrite, no new doc files. The doc tree uses subdirectories with descriptive filenames (docs/core-concepts/runtime-engine.md, docs/security/secret-management.md, etc.) but the README linked to canonical flat names that don't exist on disk. Each broken link is updated to point at the existing target.

Mapping applied (16 mechanical, 3 resolved by matching each row's description-cell text to the closest existing doc):

Was	Now
docs/quickstart.md	docs/getting-started/quick-start.md
docs/installation.md	docs/getting-started/installation.md
docs/architecture.md	docs/core-concepts/how-forge-works.md
docs/skills.md	docs/skills/writing-custom-skills.md
docs/tools.md	docs/core-concepts/tools-and-builtins.md
docs/runtime.md	docs/core-concepts/runtime-engine.md
docs/memory.md	docs/core-concepts/memory-system.md
docs/channels.md	docs/core-concepts/channels.md
docs/scheduling.md	docs/core-concepts/scheduling.md
docs/security/egress.md	docs/security/egress-control.md
docs/security/secrets.md	docs/security/secret-management.md
docs/security/signing.md	docs/security/build-signing.md
docs/commands.md	docs/reference/cli-reference.md
docs/configuration.md	docs/reference/forge-yaml-schema.md
docs/dashboard.md	docs/reference/web-dashboard.md
docs/deployment.md	docs/deployment/kubernetes.md
docs/hooks.md	docs/core-concepts/hooks.md
docs/plugins.md	docs/reference/framework-plugins.md
docs/command-integration.md	docs/reference/command-integration.md

The four already-working links (docs/security/overview.md, docs/security/guardrails.md, CONTRIBUTING.md, CODE_OF_CONDUCT.md) are unchanged.

Regression protection — new CI check

.github/workflows/docs-links.yaml adds a small job that walks every relative .md link in README.md and fails the build if any resolve to a missing file. Path-filtered (README.md, docs/**, the workflow itself) so it only runs on PRs that could plausibly break it.

on:
  push:    { branches: [main, develop], paths: ["README.md", "docs/**", "..."] }
  pull_request: { branches: [main, develop], paths: ["README.md", "docs/**", "..."] }

A pure-Python step — no Go toolchain needed for what's really a text check.

Audit results

$ python3 link-check.py
before:  BROKEN: 21,  OK: 4
after:   BROKEN: 0,   OK: 25

The script in the workflow is the same one used to produce these numbers.

Acceptance criteria from #58

• Every relative .md link in README.md resolves to a file that exists on main.
• A CI check catches future README link breakage before merge.
• Clicking from the README to each linked doc lands on a non-404 page.

Out of scope (intentional, per the issue)

The 34 in-doc cross-references that share the same canonical-name mismatch (e.g. docs/core-concepts/runtime-engine.md links to tools.md which doesn't exist by that name) are not fixed here. The README is the priority because it's the public discovery surface; the in-doc fixes belong in a separate follow-up so this PR stays narrowly README-focused. The CI check landed here is README-only by design — extending it to all docs is a clean next step but a separate decision.