Allow an explicit demand-bounded exception to percentage-only capacity refusal

[chubes4]

Problem

Worktree admission currently uses the maximum of absolute and percentage free-space floors. On a large filesystem, the percentage floor can require roughly 93 GiB free and block a roughly 40 MiB worktree even while 26-48 GiB and healthy inode capacity remain available.

The unrestricted --force escape hatch works, but it discards more policy than this case requires. --remediate-capacity can also be disproportionate when the requested demand is tiny and the refusal is percentage-only.

Acceptance

• Add an explicit admission mode limited to refusals caused solely by the percentage-byte floor.
• Preserve absolute-byte, inode, lock, freshness, lifecycle, and containment protections.
• Require projected demand below a small configured ceiling and a trusted demand source.
• Return typed evidence naming the waived percentage floor, retained hard floors, projected post-create capacity, and explicit operator intent.
• Keep --force as the unrestricted override and --remediate-capacity as reclaim-and-retry.
• Cover large-filesystem/small-worktree admission and reject large or unknown demand.

Related completed work: #1044, #1045, #1000, and #1118. None provides a narrow percentage-only exception.

AI assistance

OpenAI GPT-5.6 Sol via OpenCode analyzed DMC capacity evidence from a blocked worktree creation, reviewed existing capacity trackers, and drafted this report. Chris Huber reviewed and remains responsible.