docs: add a docs/ directory with a user guide and developer documentation (#128)

Closes #127

## What

Adds a `docs/` directory — the repo had none — split into the two
categories the issue asked for.

**`docs/user-guide/`** (11 pages) — installation, authentication,
workspaces & profiles, Slack links & timestamps, one page per command
group (conversations, messages, search, saved, canvas), scripting & JSON
output, troubleshooting.

**`docs/development/`** (6 pages) — setup, architecture, project
structure, testing, build & release, and a worked example of adding a
command.

`docs/README.md` is the index; each category has its own index too.

## Why

Everything lived in a single ~20 KB `README.md` covering install, four
auth methods, every command, configuration, development and
troubleshooting in one scroll. A user could not find the one command
they needed, and a contributor got nothing about how the codebase is
actually put together.

## Notable content

The developer pages write down what was previously only visible in
source comments:

- the dual-auth dispatch in `SlackClient.request()`, and a table of the
five places the two auth types genuinely diverge (`createDraft`,
`listSavedItems`, `searchModules`, `getUnreadCounts`, `fetchMessage`)
with the reason for each;
- the profile-key derivation and resolution rules in `workspaces.ts`,
including the backward-compatibility constraints on legacy config files;
- the CDP-based `login-auto` capture, why it is hand-rolled rather than
Playwright, and the security invariants worth preserving;
- the `writeJson()` / `process.exit()` hazard behind #73, called out in
both the architecture page and the new-command checklist;
- why Bun is pinned to 1.3.13 and why the 150 MB binary budget is a
design constraint rather than a nicety.

## README

Trimmed to an overview that links into `docs/`: header, demo, features,
installation, a short authentication summary, a quick tour,
configuration, development, contributing, and pointers for the rest.

One correctness fix while there: the README's Contributing section
described a plain fork-and-PR flow, which contradicts the issue-first /
`ready-for-pr` policy in `CONTRIBUTING.md` and `CLAUDE.md`. It now
matches.

## Verification

- Documentation only — no source or behaviour changes.
- Every relative link in `README.md` and `docs/` checked to resolve.
- `bun run type-check` clean; `bun test` 422 pass / 0 fail; `pre-commit
run --all-files` passes.