[clubanderson]

Closes #12130
Refs #12134

What

• pkg/promptsrc: maxPromptBytes = 512 KiB (memory bound) → DefaultMaxPromptBytes = 64 KiB (token budget), a named exported constant. Measured against every shipped template in src/policies/ and src/pkg/policies/defaults/ (83 files): the largest is scanner-automerge.md at 27,890 bytes, so 64 KiB is 2.3× headroom. TestShippedTemplatesFitUnderCap walks both trees and fails if any template drops under 2× headroom, naming the file.
• Override: HIVE_PROMPT_SOURCE_MAX_BYTES (bytes, or KiB/MiB suffix); unparseable/non-positive keeps the default. Resolver.SetMaxBytes for programmatic override.
• Oversize → reject whole, never truncate. Resolve logs prompt from GitHub exceeds size cap; REJECTED whole (not truncated), falling back with repo, path, ref, size_bytes, cap_bytes, override_env, then serves the last-known-good cached copy if any, else returns Ok=false, Source:"oversize" so the scheduler takes the next resolution step (kick_template → pack → convention → embedded default). The rejected body is never cached. FetchOnce returns a wrapped ErrPromptTooLarge naming source, size and cap, so the dashboard's bake/save flow refuses rather than writing a cut-off policy to /data/policies.
• Docs: operator-reference.md (Notable fields), env-vars.md, agent-configuration.md; changelog fragment.

Constraints from the issue

• Cap above shipped and real templates: yes, with a test pinning the margin.
• Silent partial policy is worse than no remote policy: no truncation path remains; the two old *_TruncatesOversizeBody tests are replaced by reject/fallback/log tests.

Validation

gofmt -l clean on touched files; go build ./..., go vet ./pkg/promptsrc/... ./pkg/dashboard/ ./pkg/scheduler/, go test ./pkg/promptsrc/... ./pkg/config/... green locally.

cc @mrbobbytables

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[github-actions]

Open PR overlap check

Base branch: v5

Conflicts are checked with every PR merged onto the current tip of v5, so a PR that only needs a rebase is not reported as conflicting with its siblings.

PR	Title	Relation	Files
#12157	🌱 tokens: measure kick size per kick; keep kick_limits defaults	same files	src/docs/operator-reference.md
#12156	🌱 tokens: cap AGENTS.md injection at section boundaries with a visible warning	same files	src/docs/operator-reference.md
#12150	🌱 tokens: byte caps for knowledge primer and Context7 content, with visible truncation	same files	src/docs/operator-reference.md
#12025	✨ keep unstick steps running in conserve API budget mode	same files	src/docs/operator-reference.md

[hivecommons-hive]

merged by Hive trusted-author auto-merge: author @clubanderson holds role owner and GitHub permission admin on repo; CI green at c9f1586.

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