docs(README): add newcomer orientation, ZSH plugin reference, and sync CLI docs

[ahundt]

Adds onboarding context and documents the ZSH plugin system, which was not
covered in the README. Also updates the CLI reference to match the current
binary and clarifies a few configuration details.

What's added

• "How Forge Works: Three Modes" section orienting newcomers to the three
  distinct usage patterns: Interactive TUI, One-Shot CLI (-p), and ZSH Plugin.
  Includes a note that forge conversation resume <id> opens the interactive
  TUI (not a one-shot command), which is a common point of confusion.
• "ZSH Plugin: The : Prefix System" section with full coverage of agents
  (forge/sage/muse, aliases :ask/:plan), prompt sending, file attachment via
  @ + Tab, conversation management (:new, :conversation, :clone,
  :rename, :retry, :copy, :dump, :compact), git integration
  (:commit, :commit-preview), shell tools (:suggest, :edit),
  session-only vs persistent config options, skills (built-in and custom),
  AGENTS.md, custom agents/commands, semantic search/workspace, and a
  quick-reference table of all : commands.
• "Subcommands" section listing all forge subcommands by category.

What's updated

• Command-Line Options table: reflects current flags (--conversation-id,
  --agent, -C/--directory, --sandbox); removes flags not present in the
  current CLI (-c, -w, -r).
• MCP section: replaces commands from an earlier version (add, add-json,
  get) with the current subcommands (import, show, remove, reload);
  makes config paths explicit (./.mcp.json for project-local,
  ~/forge/.mcp.json for global).
• ZSH plugin env var comment: corrected from # prefix to : prefix.
• Global config path: corrected from ~/.forge/.forge.toml to
  ~/forge/.forge.toml (source: crates/forge_infra/src/env.rs line 27-29,
  h.join("forge")).
• Three minor grammar fixes (missing comma, pronoun clarity, sentence structure).

[CLAassistant]

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1 out of 2 committers have signed the CLA.

✅ tusharmath
❌ ahundt
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[tusharmath]

Many many thanks!