Reconcile README with current source tree

[glenjbarber]

Summary

• Updates the implementation-status tables to reflect the packages that now exist (cmd/orcli, internal/config, internal/keychain, internal/mcp, internal/saved, internal/tools, internal/tui, and the various API client packages), and keeps internal/bootstrap/internal/complete listed as not yet written, since they genuinely are not.
• Removes the stale "no cmd/orcli yet, so no binary" claim and the dangling DESIGN.md/AGENTS.md references (neither file exists in the repo).
• Corrects the package dependency diagram to show cmd/orcli as the only package that imports both tui and tools, wiring them together at runtime rather than either importing the other.
• Rewrites the "Rendering" section to describe tcell's actual, narrow role (terminal init, terminfo capability lookup, input decoding) rather than claiming raw syscall with no terminal library, while still noting that screen painting is direct raw writes to stdout, not tcell's own Draw/Show path. Notes that internal/tui's fully tview/tcell-rendered code path (startScreenMode) has no callers and is not reached at runtime.
• Removes the stale "This is the one package that exists." line under "API client".

Fixes #30

Test plan

• make test (all packages pass)
• Manual read-through of the updated README against the current source tree

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https://claude.ai/code/session_01Nxr4mW9rMNzn9kThjdJpZt

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