WIP: add context budget UI groundwork

[YumaKakuya]

Summary

• Add shared context budget helper, assistant context snapshots, footer groundwork, and /context dialog with /ctx alias.
• Wire route-aware context metadata into processor, overflow, and compaction paths.
• Preserve this as WIP because QA found blocker findings H1-H4.

Verification

• cd /home/yuma/hatch-v3/packages/opencode && bun typecheck passed.
• QA verdict: FAIL.

Known Blockers

• H1: snapshot denominator diverges from live/overflow denominator.
• H2: usable calculation ignores reservedCompaction for safe input path.
• H3: footer fallback infers auth route from provider_auth available methods.
• H4: footer fallback hardcodes 32_000.

Notes

• This PR intentionally preserves the WIP for follow-up Senior remediation and QA/Reviewer review.

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• fix: or fix(scope): bug fix
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