docs: add filesystem isolation plan

[cboone]

Lay out five approaches to container filesystem isolation, from
the existing read-only worktrees mode through APFS copy-on-write
cloning, with intermediate isolated+ephemeral, named volume, and
host-side persistent directory options. Includes pros/cons, scope,
marketing blurbs, and a four-phase implementation roadmap that
builds toward the v1.2 APFS cloning target.

https://claude.ai/code/session_01Tds7kuhfb8X1tfVKTVP15q

[Copilot]

Pull request overview

Adds a new design/roadmap document that surveys filesystem isolation options for bopca—from the current read-only workspace + worktrees setup through APFS copy-on-write cloning—and proposes a phased implementation roadmap toward the v1.2 isolation target.

Changes:

• Document five filesystem isolation approaches with pros/cons and suggested CLI/config surfaces.
• Add a comparison section (isolation/startup cost/best-for) to clarify tradeoffs.
• Propose a four-phase implementation plan culminating in APFS CoW cloning with diff/accept/reset workflow.

[Copilot]

The markdown table rows start with ||, which creates an extra empty first column in GitHub-flavored markdown. Use a single leading | for each row. Also, in this table Approach 5 bind-mounts a host-side directory, so the agent can read/write host files in that managed workspace; update the Approach 5 cells (or clarify the column headings if they specifically mean the original repo).

Suggested change

	| 5. Isolated + host directory | No | No | Yes | Yes |
	| 5. Isolated + host directory | Yes (managed dir) | Yes (managed dir) | Yes | Yes |

[Copilot]

The table rows start with ||, which renders an unintended empty first column. Use a single leading | for each row.

[Copilot]

The table rows start with ||, which renders an unintended empty first column. Use a single leading | for each row.