docs: add development guide and fix broken documentation links

[kranthik10]

Summary

This PR resolves several broken documentation references across the repository and adds a comprehensive development guide for contributors.

Changes

• Added docs/development.md: Created the missing development guide referenced in root README.md and README.zh-CN.md. It covers:
  • Monorepo workspace architecture (cli, sdk, common, agents, packages/)
  • Prerequisites (Bun v1.3.14+, Node v22+, tmux)
  • Development setup (bun install, bun start-cli, bun run dev:freebuff)
  • Build commands (bun run build:sdk, bun run build:freebuff)
  • Testing workflows and pre-PR checklist
• Fixed cli/README.md: Updated broken tmux.knowledge.md link to point to scripts/tmux/README.md and docs/testing.md.
• Fixed cli/src/__tests__/README.md: Updated broken ../../tmux.knowledge.md reference to point to scripts/tmux/README.md and docs/testing.md.

Verification

• Verified all relative markdown links resolve to valid files.
• Built SDK with bun run build:sdk to ensure no build regressions.

[codebuff-team]

Thanks for tackling the broken tmux.knowledge.md references in cli/README.md and cli/src/__tests__/README.md — that's a legitimate, low-risk fix and worth porting on its own.

The new docs/development.md, however, needs more care before landing:

1. Both fixed links now point to docs/testing.md, but this PR doesn't add or reference that file elsewhere as already existing. If docs/testing.md doesn't exist in the tree, you've just swapped one broken link for another — please confirm it exists (or add it) before this is portable.
2. Several specifics in the new guide — bun start-cli, bun run dev:freebuff, bun run build:freebuff, bun run ci — should be checked against the actual package.json scripts. If any of these don't match real script names, the guide will mislead new contributors, which is worse than no guide.
3. The workspace table (agent-runtime, code-map, llm-providers, etc.) needs verifying against the current directory layout — some of these paths read like plausible guesses rather than confirmed structure.

Since this is a docs-only PR, the bar is really just factual accuracy — there's no code to review, so every claim needs to be double-checked against the repo you're actually looking at. Please re-verify the links and script names, and mention in the PR description exactly how you validated each one (e.g., "ran bun run dev:freebuff locally and confirmed it starts"). Happy to look again once that's tightened up.

[kranthik10]

Thanks for the thorough review and feedback! I've gone through and validated every claim against the repository:

1. Verified docs/testing.md

• Confirmed docs/testing.md is present on main (29KB testing guide).
• Checked both relative links to ensure they resolve:
  • In cli/README.md: ../docs/testing.md
  • In cli/src/__tests__/README.md: ../../../docs/testing.md

2. Validated All Scripts Locally

I tested and validated each command directly against root package.json:

• bun run build:sdk: Compiled @codebuff/sdk (ESM/CJS bundles, type definitions, and WASM bindings).
• bun run build:freebuff: Produced cli/bin/freebuff executable.
• bun run ci: Successfully ran bun run build:sdk && bun run build:freebuff with exit code 0.
• bun run dev:freebuff / bun start-cli: Verified entry point execution with configured client environment.
• Added an explicit Available Root Scripts mapping table in docs/development.md and added a note regarding .env.local requirements for local interactive dev.

3. Verified Monorepo Workspaces

• Cross-referenced the architecture table against all 10 entries in package.json's "workspaces" field (agents, cli, common, evals, freebuff, packages/agent-runtime, packages/code-map, packages/llm-providers, scripts/tmux, sdk), including adding evals/ (buffbench).

The branch has been updated with these refinements. Ready for another review!