docs: add agent friction to concepts glossary

Author: ppiankov

docs/concepts.md

@@ -11,6 +11,7 @@
 | **Context deadlock** | Terminal state where a session is too large to continue (API rejects new messages) and too large to compact (compaction is itself an API call). See [Context Deadlock](deadlock.md). |
 | **Signal / Noise** | Signal = content that contributes to productive reasoning. Noise = waste that fills the session file without value: progress metadata, duplicate file reads, failed retries, decorative separators, cross-repo tangents. Some noise is never sent to the API (progress, snapshots). Some is sent but adds nothing (stale reads, tangents). |
 | **Noise multiplier** | `recoverable_noise / current_context`. How much removable junk the session is carrying relative to active context. 0.1x = near-perfect trace. 0.4x = mostly clean. 1.5x = more garbage than signal. 3x+ = session drowning in noise. Technical debt for context. The number engineers instantly understand - a 1.5x multiplier means the model spends more compute rereading waste than reasoning. Visible in stats output and JSON as `noise_multiplier`. |
+| **Agent friction** | Token waste from coordination failures, not reasoning: failed file reads retried against wrong paths, parallel operations that cancel each other, oversized reads that hit limits and force repeated attempts, stale binary paths after version updates ([anthropics/claude-code#32352](https://github.com/anthropics/claude-code/issues/32352)), and low-value narration between tool calls. These tokens buy no exploration or decisions but compound via the same re-read snowball as abandoned reasoning branches. A distinct cost category from decision cost (productive exploration) and snowball cost (dead exploration compounding). |
 | **Vector health score** | A-F grade measuring signal-to-noise ratio. A = >95% signal. F = <20% signal. |
 | **Input purity score (IPS)** | 0-100 metric measuring how much incoming tool output is already purified before entering context. 100 = all input pre-compressed, 0 = all input raw. Computed by classifying tool result entries against known compressible patterns (git status, ls, test/lint output) and estimating saveable tokens. The leading indicator - low IPS predicts future noise ratio increase. Shipped - visible in status line as `ips:N`. |
 | **Input purification** | Compressing or filtering command output before it enters the context window. Prevents dilution at the source - tokens that never enter context don't accumulate re-read tax. Tools like [RTK](https://github.com/rtk-ai/rtk) implement this as a transparent CLI proxy (e.g., `git status` output compressed from 10 lines to 1). Complementary to post-hoc cleanup: input purification reduces what goes in, vector sharpening cleans what's already there. Two stages of the same anti-dilution pipeline. |