Docs: Clarify CLI modes and context length

[amypoolside]

Summary

• Split approval modes and Build/Plan agent modes into separate README sections.
• Document how to enter and leave Plan mode, including the absence of a Plan-mode startup flag.
• Clarify that POOLSIDE_STANDALONE_CONTEXT_LENGTH controls automatic compaction thresholds and does not change the model server context window.

Verification

• git diff --check
• Reviewed the documented commands and environment-variable behavior against the v1.0.15 implementation.

[Copilot]

Pull request overview

This PR updates the README to better distinguish between (1) tool-approval behavior and (2) the agent’s build/plan operating mode, and clarifies what POOLSIDE_STANDALONE_CONTEXT_LENGTH actually controls in standalone mode.

Changes:

• Split the former “Modes” documentation into separate “Approval modes” and “Agent modes” sections, with updated TOC entries.
• Document how to enter/exit Plan mode after startup (and that there is no Plan-mode startup flag).
• Clarify that POOLSIDE_STANDALONE_CONTEXT_LENGTH affects automatic compaction thresholds rather than the model server’s actual context window.

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[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

Suppressed comments (1)

README.md:75

• The /mode argument placeholder is ambiguous: the table uses an ID column (e.g. accept-edits, always-allow), but /mode <approval-mode> could be interpreted as the human-readable name (e.g. “Always ask”). Consider naming the placeholder to match the table’s ID terminology to avoid confusion.

Press `Shift+Tab` to cycle through approval modes, or use `/mode <approval-mode>` to switch directly.